In Doxy 3.x we used Advance Custom Field instead of CMB2. So if you want to use the 3.x  version in your existing wordpress website where have 2.x installed, you have to migrate the docs manually. Otherwise you will lose your previous docs. But don’t worry, we have simple migration tricks to migrate Doxy 2.x to Doxy 3.x

If you are unable to do that yourself. Feel free contact us and we will charge a very little fee to do migration work for you.

Here following the stepts :
Step 1 : Take a full backup of your current website where have doxy 2.x installed.

Step 2 : Export the docs from wordpress admin dashboard > Tools > Export > docs and then download the xml file.

Step 3 : Now install Doxy 3.x version and update required plugins. ( Don’t forget to install required plugin : Advance Custom Field ), we drooped cmb2 plugin in that new version.

Step 4 : Open the downloaded xml file in any Editor. ( Examle : demo2.xml )

<?xml version="1.0" encoding="UTF-8"?>
<!-- This is a WordPress eXtended RSS file generated by WordPress as an export of your site. -->
<!-- It contains information about your site's posts, pages, comments, categories, and other content. -->
<!-- You may use this file to transfer that content from one site to another. -->
<!-- This file is not intended to serve as a complete backup of your site. -->
<!-- To import this information into a WordPress site follow these steps: -->
<!-- 1. Log in to that site as an administrator. -->
<!-- 2. Go to Tools: Import in the WordPress admin panel. -->
<!-- 3. Install the "WordPress" importer from the list. -->
<!-- 4. Activate & Run Importer. -->
<!-- 5. Upload this file using the form provided on that page. -->
<!-- 6. You will first be asked to map the authors in this export file to users -->
<!--    on the site. For each author, you may choose to map to an -->
<!--    existing user on the site or to create a new user. -->
<!-- 7. WordPress will then import each of the posts, pages, comments, categories, etc. -->
<!--    contained in this file into your site. -->
<!-- generator="WordPress/5.2.1" created="2019-06-13 03:57" -->
<rss xmlns:content="http://purl.org/rss/1.0/modules/content/" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:excerpt="http://wordpress.org/export/1.2/excerpt/" xmlns:wfw="http://wellformedweb.org/CommentAPI/" xmlns:wp="http://wordpress.org/export/1.2/" version="2.0">
   <channel>
      <title>My WordPress Website</title>
      <link>http://themeix.com/doxy20</link>
      <description>Just another WordPress site</description>
      <pubDate>Thu, 13 Jun 2019 03:57:13 +0000</pubDate>
      <language>en-US</language>
      <wp:wxr_version>1.2</wp:wxr_version>
      <wp:base_site_url>http://themeix.com/doxy20</wp:base_site_url>
      <wp:base_blog_url>http://themeix.com/doxy20</wp:base_blog_url>
      <wp:author>
         <wp:author_id>1</wp:author_id>
         <wp:author_login><![CDATA[siteadmin]]></wp:author_login>
         <wp:author_email><![CDATA[[email protected]]]></wp:author_email>
         <wp:author_display_name><![CDATA[siteadmin]]></wp:author_display_name>
         <wp:author_first_name />
         <wp:author_last_name />
      </wp:author>
      <generator>https://wordpress.org/?v=5.2.1</generator>
      <item>
         <title>Post example with chapter</title>
         <link>http://themeix.com/doxy20/index.php/docs/post-example-with-chapter/</link>
         <pubDate>Thu, 13 Jun 2019 03:51:06 +0000</pubDate>
         <dc:creator><![CDATA[siteadmin]]></dc:creator>
         <guid isPermaLink="false">http://themeix.com/doxy20/?post_type=docs&p=11</guid>
         <description />
         <content:encoded />
         <excerpt:encoded />
         <wp:post_id>11</wp:post_id>
         <wp:post_date><![CDATA[2019-06-13 03:51:06]]></wp:post_date>
         <wp:post_date_gmt><![CDATA[2019-06-13 03:51:06]]></wp:post_date_gmt>
         <wp:comment_status><![CDATA[closed]]></wp:comment_status>
         <wp:ping_status><![CDATA[closed]]></wp:ping_status>
         <wp:post_name><![CDATA[post-example-with-chapter]]></wp:post_name>
         <wp:status><![CDATA[publish]]></wp:status>
         <wp:post_parent>0</wp:post_parent>
         <wp:menu_order>0</wp:menu_order>
         <wp:post_type><![CDATA[docs]]></wp:post_type>
         <wp:post_password />
         <wp:is_sticky>0</wp:is_sticky>
         <wp:postmeta>
            <wp:meta_key><![CDATA[_edit_last]]></wp:meta_key>
            <wp:meta_value><![CDATA[1]]></wp:meta_value>
         </wp:postmeta>
         <wp:postmeta>
            <wp:meta_key><![CDATA[_doxy_short_desc]]></wp:meta_key>
            <wp:meta_value><![CDATA[Short Description For that post]]></wp:meta_value>
         </wp:postmeta>
         <wp:postmeta>
            <wp:meta_key><![CDATA[_doxy_doxy_doc_chapter]]></wp:meta_key>
            <wp:meta_value><![CDATA[a:2:{i:0;a:2:{s:18:"_doxy_chapter_name";s:20:"Chapter 1 title here";s:17:"_doxy_doc_chapter";s:56:" Chapter 1 Content Here";}i:1;a:2:{s:18:"_doxy_chapter_name";s:20:"Chapter 2 title Here";s:17:"_doxy_doc_chapter";s:1428:"Chapter 2 Content here";}}]]></wp:meta_value>
         </wp:postmeta>
      </item>
   </channel>
</rss>

Step 5 : Now find the following code

<wp:postmeta>
<wp:meta_key><![CDATA[_edit_last]]></wp:meta_key>
<wp:meta_value><![CDATA[1]]></wp:meta_value>
</wp:postmeta>

Now add the following code after that,

<!-- Do you want to show chapter in Doc? (Start) -->		
<wp:postmeta>
<wp:meta_key><![CDATA[show_chapter_in_doc]]></wp:meta_key>
<wp:meta_value><![CDATA[yes]]></wp:meta_value>
</wp:postmeta>		
<wp:postmeta>
<wp:meta_key><![CDATA[_show_chapter_in_doc]]></wp:meta_key>
<wp:meta_value><![CDATA[field_5cfe6e5440f1f]]></wp:meta_value>
</wp:postmeta>
<!--  Do you want to show chapter in Doc?  (End) -->	

<!-- Add ACF Key For Short Description (Start) -->
<wp:postmeta>
<wp:meta_key><![CDATA[__doxy_short_desc]]></wp:meta_key>
<wp:meta_value><![CDATA[field_5cfa012d625bf]]></wp:meta_value>
</wp:postmeta>
<!-- Add ACF Key For Short Description (End) -->

<!-- Add ACF Key For Chapter Field (End) -->	
<wp:postmeta>
<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter]]></wp:meta_key>
<wp:meta_value><![CDATA[2]]></wp:meta_value>
</wp:postmeta>
<wp:postmeta>
<wp:meta_key><![CDATA[__doxy_doxy_doc_chapter]]></wp:meta_key>
<wp:meta_value><![CDATA[field_5cfa01d7625c0]]></wp:meta_value>
</wp:postmeta>
<!-- Add ACF Key For Short Description (End) -->

Step 6 : Now need to repair chapters ( Chapter Name and Contents ). Here is the 2.x code for the chapters :

<wp:postmeta>
<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter]]></wp:meta_key>
<wp:meta_value><![CDATA[a:2:{i:0;a:2:{s:18:”_doxy_chapter_name”;s:20:”Chapter 1 title here”;s:17:”_doxy_doc_chapter”;s:56:” Chapter 1 Content Here”;}i:1;a:2:{s:18:”_doxy_chapter_name”;s:20:”Chapter 2 title Here”;s:17:”_doxy_doc_chapter”;s:1428:”Chapter 2 Content here”;}}]]></wp:meta_value>
</wp:postmeta>

we need to replace it with our structured wp:postmeta for 3.x support. There have 2 chapter name and 2 chapter content in above code. So we need to create 4  wp:postmeta. Here is the code for the first one :

<wp:postmeta>
<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter_0__doxy_chapter_name]]></wp:meta_key>
<wp:meta_value><![CDATA[Chapter 1 title here]]></wp:meta_value>
</wp:postmeta>

<wp:postmeta>
<wp:meta_key><![CDATA[__doxy_doxy_doc_chapter_0__doxy_chapter_name]]></wp:meta_key>
<wp:meta_value><![CDATA[field_5cfa0267625c2]]></wp:meta_value>
</wp:postmeta>	

For the next chapter’s name you should just increment the number 0 to 1,2,3, 4 etc ( Example : _doxy_doxy_doc_chapter_1__doxy_chapter_name  , __doxy_doxy_doc_chapter_1__doxy_chapter_name )
**and 
<![CDATA[field_5cfa0267625c2]]>** is the common meta_value for all the doc chapter name.

So if you have 2 chapter with 2 chapter name then the code will be as bellow :

<wp:postmeta>
<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter_0__doxy_chapter_name]]></wp:meta_key>
<wp:meta_value><![CDATA[Chapter 1 title here]]></wp:meta_value>
</wp:postmeta>
<wp:postmeta>
<wp:meta_key><![CDATA[__doxy_doxy_doc_chapter_0__doxy_chapter_name]]></wp:meta_key>
<wp:meta_value><![CDATA[field_5cfa0267625c2]]></wp:meta_value>
</wp:postmeta>		
<wp:postmeta>
<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter_1__doxy_chapter_name]]></wp:meta_key>
<wp:meta_value><![CDATA[Chapter 2 title here]]></wp:meta_value>
</wp:postmeta>
<wp:postmeta>
<wp:meta_key><![CDATA[__doxy_doxy_doc_chapter_1__doxy_chapter_name]]></wp:meta_key>
<wp:meta_value><![CDATA[field_5cfa0267625c2]]></wp:meta_value>
</wp:postmeta>	

Now for the chapter content we need to add the following code :

<wp:postmeta>
<wp:meta_key><![CDATA[__doxy_doxy_doc_chapter_0__doxy_doc_chapter]]></wp:meta_key>
<wp:meta_value><![CDATA[field_5cfa0359625c3]]></wp:meta_value>
</wp:postmeta>
<wp:postmeta>
<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter_0__doxy_doc_chapter]]></wp:meta_key>
<wp:meta_value><![CDATA[Chapter 1 Content Here]]></wp:meta_value>
</wp:postmeta>
<wp:postmeta>
<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter_1__doxy_doc_chapter]]></wp:meta_key>
<wp:meta_value><![CDATA[Chapter 2 Content here]]></wp:meta_value>
</wp:postmeta>
<wp:postmeta>
<wp:meta_key><![CDATA[__doxy_doxy_doc_chapter_1__doxy_doc_chapter]]></wp:meta_key>
<wp:meta_value><![CDATA[field_5cfa0359625c3]]></wp:meta_value>
</wp:postmeta>	

Same rule applied as you used for Chapter Name.
<![CDATA[field_5cfa0359625c3]]> is the default meta_value for incremental meta_key of docs chapter content.

So here is the final code for doxy 3.x

<?xml version="1.0" encoding="UTF-8" ?>
<!-- This is a WordPress eXtended RSS file generated by WordPress as an export of your site. -->
<!-- It contains information about your site's posts, pages, comments, categories, and other content. -->
<!-- You may use this file to transfer that content from one site to another. -->
<!-- This file is not intended to serve as a complete backup of your site. -->

<!-- To import this information into a WordPress site follow these steps: -->
<!-- 1. Log in to that site as an administrator. -->
<!-- 2. Go to Tools: Import in the WordPress admin panel. -->
<!-- 3. Install the "WordPress" importer from the list. -->
<!-- 4. Activate & Run Importer. -->
<!-- 5. Upload this file using the form provided on that page. -->
<!-- 6. You will first be asked to map the authors in this export file to users -->
<!--    on the site. For each author, you may choose to map to an -->
<!--    existing user on the site or to create a new user. -->
<!-- 7. WordPress will then import each of the posts, pages, comments, categories, etc. -->
<!--    contained in this file into your site. -->

	<!-- generator="WordPress/5.2.1" created="2019-06-13 03:57" -->
<rss version="2.0"
	xmlns:excerpt="http://wordpress.org/export/1.2/excerpt/"
	xmlns:content="http://purl.org/rss/1.0/modules/content/"
	xmlns:wfw="http://wellformedweb.org/CommentAPI/"
	xmlns:dc="http://purl.org/dc/elements/1.1/"
	xmlns:wp="http://wordpress.org/export/1.2/"
>

<channel>
	<title>My WordPress Website</title>
	<link>http://themeix.com/doxy20</link>
	<description>Just another WordPress site
</description>
	<pubDate>Thu, 13 Jun 2019 03:57:13 +0000</pubDate>
	<language>en-US</language>
	<wp:wxr_version>1.2</wp:wxr_version>
	<wp:base_site_url>http://themeix.com/doxy20</wp:base_site_url>
	<wp:base_blog_url>http://themeix.com/doxy20</wp:base_blog_url>

		<wp:author><wp:author_id>1</wp:author_id><wp:author_login><![CDATA[siteadmin]]></wp:author_login><wp:author_email><![CDATA[[email protected]]]></wp:author_email><wp:author_display_name><![CDATA[siteadmin]]></wp:author_display_name><wp:author_first_name><![CDATA[]]></wp:author_first_name><wp:author_last_name><![CDATA[]]></wp:author_last_name></wp:author>

				
	<generator>https://wordpress.org/?v=5.2.1</generator>

		<item>
		<title>Post example with chapter</title>
		<link>http://themeix.com/doxy20/index.php/docs/post-example-with-chapter/</link>
		<pubDate>Thu, 13 Jun 2019 03:51:06 +0000</pubDate>
		<dc:creator><![CDATA[siteadmin]]></dc:creator>
		<guid isPermaLink="false">http://themeix.com/doxy20/?post_type=docs&p=11</guid>
		<description></description>
		<content:encoded><![CDATA[]]></content:encoded>
		<excerpt:encoded><![CDATA[]]></excerpt:encoded>
		<wp:post_id>11</wp:post_id>
		<wp:post_date><![CDATA[2019-06-13 03:51:06]]></wp:post_date>
		<wp:post_date_gmt><![CDATA[2019-06-13 03:51:06]]></wp:post_date_gmt>
		<wp:comment_status><![CDATA[closed]]></wp:comment_status>
		<wp:ping_status><![CDATA[closed]]></wp:ping_status>
		<wp:post_name><![CDATA[post-example-with-chapter]]></wp:post_name>
		<wp:status><![CDATA[publish]]></wp:status>
		<wp:post_parent>0</wp:post_parent>
		<wp:menu_order>0</wp:menu_order>
		<wp:post_type><![CDATA[docs]]></wp:post_type>
		<wp:post_password><![CDATA[]]></wp:post_password>
		<wp:is_sticky>0</wp:is_sticky>
		<wp:postmeta>
		<wp:meta_key><![CDATA[_edit_last]]></wp:meta_key>
		<wp:meta_value><![CDATA[1]]></wp:meta_value>
		</wp:postmeta>
		
		<!-- Do you want to show chapter in Doc? (Start) -->		
		<wp:postmeta>
		<wp:meta_key><![CDATA[show_chapter_in_doc]]></wp:meta_key>
		<wp:meta_value><![CDATA[yes]]></wp:meta_value>
		</wp:postmeta>		
		<wp:postmeta>
		<wp:meta_key><![CDATA[_show_chapter_in_doc]]></wp:meta_key>
		<wp:meta_value><![CDATA[field_5cfe6e5440f1f]]></wp:meta_value>
		</wp:postmeta>
		<!--  Do you want to show chapter in Doc?  (End) -->	
		<!-- Add ACF Key For Short Description (Start) -->
		<wp:postmeta>
		<wp:meta_key><![CDATA[__doxy_short_desc]]></wp:meta_key>
		<wp:meta_value><![CDATA[field_5cfa012d625bf]]></wp:meta_value>
		</wp:postmeta>
		<!-- Add ACF Key For Short Description (End) -->	

		<!-- Add ACF Key For Chapter Field (End) -->	
		<wp:postmeta>
		<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter]]></wp:meta_key>
		<wp:meta_value><![CDATA[2]]></wp:meta_value>
		</wp:postmeta>
		<wp:postmeta>
		<wp:meta_key><![CDATA[__doxy_doxy_doc_chapter]]></wp:meta_key>
		<wp:meta_value><![CDATA[field_5cfa01d7625c0]]></wp:meta_value>
		</wp:postmeta>
		<!-- Add ACF Key For Short Description (End) -->			
		
		<wp:postmeta>
		<wp:meta_key><![CDATA[_doxy_short_desc]]></wp:meta_key>
		<wp:meta_value><![CDATA[Lorem Ipsum is simply dummy text of the printing and typesetting industry. ]]></wp:meta_value>
		</wp:postmeta>

		
		<!-- Chapters Repair (Start) -->	
		<wp:postmeta>
		<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter_0__doxy_chapter_name]]></wp:meta_key>
		<wp:meta_value><![CDATA[Chapter 1 title here]]></wp:meta_value>
		</wp:postmeta>
		<wp:postmeta>
		<wp:meta_key><![CDATA[__doxy_doxy_doc_chapter_0__doxy_chapter_name]]></wp:meta_key>
		<wp:meta_value><![CDATA[field_5cfa0267625c2]]></wp:meta_value>
		</wp:postmeta>		
		<wp:postmeta>
		<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter_1__doxy_chapter_name]]></wp:meta_key>
		<wp:meta_value><![CDATA[Chapter 2 title here]]></wp:meta_value>
		</wp:postmeta>
		<wp:postmeta>
		<wp:meta_key><![CDATA[__doxy_doxy_doc_chapter_1__doxy_chapter_name]]></wp:meta_key>
		<wp:meta_value><![CDATA[field_5cfa0267625c2]]></wp:meta_value>
		</wp:postmeta>	
				<!-- Chapter Content -->	
		<wp:postmeta>
		<wp:meta_key><![CDATA[__doxy_doxy_doc_chapter_0__doxy_doc_chapter]]></wp:meta_key>
		<wp:meta_value><![CDATA[field_5cfa0359625c3]]></wp:meta_value>
		</wp:postmeta>
		<wp:postmeta>
		<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter_0__doxy_doc_chapter]]></wp:meta_key>
		<wp:meta_value><![CDATA[Chapter 1 Content Here]]></wp:meta_value>
		</wp:postmeta>
		<wp:postmeta>
		<wp:meta_key><![CDATA[_doxy_doxy_doc_chapter_1__doxy_doc_chapter]]></wp:meta_key>
		<wp:meta_value><![CDATA[Chapter 2 Content here]]></wp:meta_value>
		</wp:postmeta>
		<wp:postmeta>
		<wp:meta_key><![CDATA[__doxy_doxy_doc_chapter_1__doxy_doc_chapter]]></wp:meta_key>
		<wp:meta_value><![CDATA[field_5cfa0359625c3]]></wp:meta_value>
		</wp:postmeta>		
		<!-- Chapters Repair (End) -->		 
</item>
</channel>
</rss>

If you are unable to do that yourself. Feel free contact us and we will charge a very little fee to do migration work for you.