Stephanie Leary

This is a utility for converting lots of posts or pages to a custom post type (or vice versa). You can limit the conversion to posts in a single category or children of specific page. You can also assign new taxonomy terms, which will be added to the posts’ existing terms.
This plugin is useful for converting many posts at once. If you’d rather do one at a time, use <a href="https://wordpress.org/extend/plugins/post-type-switcher/" rel="ugc">Post Type Switcher</a> instead.

Convert Post Types

Wordpress 4.9.5

No better plugin for converting categories to custom post types. And vice versa.

Still works great

A bulk conversion utility for post types.

Amaury Balmer

Nicolas Juen

This plugin allow to build relation between 2 custom types (posts, page, custom), very useful for manage related content on CMS type website.
A few example use cases:
<ul>
<li>manually lists of related posts</li>
<li>post series</li>
<li>rented houses connected to agency</li>
<li>etc.</li>
</ul>
Relationships are created from a settings page from the administration console.
This plugin can not do relationships with users, you should test this excellent plugin made by scribu 
https://wordpress.org/extend/plugins/posts-to-posts/

Relations Post Types

This plugin allow to build relation between 2 custom types (posts, page, custom), very useful for manage related content on CMS type website.

Convert Post Types vs Relations Post Types