Recently I went looking for a way to copy or move tags to a new taxonomy. I was a bit surprised that I couldn’t find anything. There’s no WordPress function that just does it, and the few taxonomy management plugins I found either haven’t been updated in years, or didn’t seem to work (at least not to do what I needed).
I’ve used the WordPress Category to Tags Converter on this blog before, but that only works for categories and tags, no other taxonomies. So, using that plugin as a basis I have written the Taxonomy Converter. It can copy or convert terms from their original taxonomy to another taxonomy, or even copy to multiple taxonomies. Posts associated with the original taxonomy terms remain associated with the terms in their new taxonomies too.
I’ve submitted this plugin to the WordPress plugin repository, but I’m not sure how long it will take to be approved, assuming it is approved. In the mean time you can download it here.