We'll create fresh WordPress site with Term Taxonomy Converter installed. You have 20 minutes to test the plugin after that site we'll be deleted.
Initial version of this plugin is a fork of https://wordpress.org/plugins/taxonomy-converter/.
Copy or convert terms between taxonomies.
Taxonomies include categories, tags and any other custom taxonomies that may be present with your active theme and plugins.
This plugin, allows you to copy (duplicate) or convert (move) terms from one taxonomy to another or to multiple taxonomies, while maintaining associated posts.