Sanam Maharjan

This is a plugin for converting lots of posts or pages to a custom post type (or vice versa). In the ‘convert from…’ select option you will get the lists of all the post types stores in your current database and in the ‘convert to…’ option you will get the list of all the current registered post types. In addition to this you can also convert from one taxonomy to another and the lists are populated just as that of post types.

Post Type Convertr

A bulk conversion utility for post types and taxonomies.

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.

Post Type Convertr vs Relations Post Types