You can copy or convert terms between different taxonomies like categories, tags and other custom taxonomies wither created by you manually or present with some activate plugins and theme. Taxonomy Converter is the new WordPress plugin based on Categories to Tags Converter. The plugin allows you to copy (duplicate) or convert (move) terms from one taxonomy to another or to multiple taxonomies while maintaining associated posts.

Install and activate Taxonomy Converter plugin then visit Dashboard -> Tools -> Import screen and click on Taxonomy Converter.