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Convert existing categories to tags or tags to categories, selectively.

Categories to Tags Converter

Th plugin is old but still work perfectly. I have tried it on a modern install I have, with PHP7.

Works fine, with PHP 7 too

Convert existing categories to tags or tags to categories, selectively.

Dhanendran Rajagopal

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.

Term Taxonomy Converter

Copy or convert terms between taxonomies.

Categories to Tags Converter vs Term Taxonomy Converter