kristarella

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, based on Categories to Tags Converter, allows you to copy (duplicate) or convert (move) terms from one taxonomy to another or to multiple taxonomies, while maintaining associated posts.

Taxonomy Converter

For starters, the plugin has deprecated code and produces errors on PHP7. Looks like the code hasn't really been touched all that much in many, many years.

The biggest problem is that when the terms are converted, the term descriptions are removed.

Doesn't include term descriptions

Copy or convert terms between taxonomies.

WordPress.org

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.

Taxonomy Converter vs Categories to Tags Converter