Convert some categories to a custom taxonomy

Category and Tag are two default WordPress taxonomies which help you to group posts together easily.
Assuming you have these categories:

• Category A
  • Subcategory 1a
  • Subcategory 2a
• Category B
  • Subcategory 1b
  • Subcategory 2b
  • Subcategory 3b

and want to convert Category B and its child categories to a new taxonomy.

I. Register a new taxonomy

First, you need to register a new taxonomy, using the register_taxonomy() function, or using another plugin (Custom Post Type UI, Toolset Types, etc.).

II. Convert category to new taxonomy

There are 2 possible ways to convert:

1. Convert automatically
   Please install and use this plugin Taxonomy Switcher.
2. Convert manually
   Please follow below steps:

• Add terms to the new taxonomy

  Add Subcategory 1b, Subcategory 2b, Subcategory 3b to new taxonomy

• Move posts from a child category to new taxonomy term

  The quickest way is:

  + In WordPress admin Posts page, select a specific category (for example: Subcategory 1b), then click “Filter” button to show all matching posts:

  + Select the top checkbox, select the “Edit” option in the “Bulk Actions” dropdown, then click “Apply” button:
  

  + Select the desired term of the new taxonomy:

  

  then click the “Update” button.

  Repeat this step for each child category.

• Remove child categories (optionally)

  After above steps, you might remove child categories which are already replaced by new taxonomy.

Best regards,