Sometimes you need to copy or convert terms between taxonomies. For instance, you are using some tag for a long time and now you want to transform it into a category so that you can provide better filters by creating its sub-categories.
Taxonomies is the term used to refer to categories, tags, and any other custom taxonomies that may be present within your active theme or plugins.
Taxonomy Converter is a free WordPress plugin, based on Categories to Tags Converter. It allows you to copy (duplicate) or convert or move terms between taxonomies.
You can do this between two or multiple taxonomies while maintaining associated posts. Simply install and activate Taxonomy Converter then visit Tools -> Import -> Taxonomy Converter page and you would be able to easily convert categories and tags on your website.