As long as you are willing to do this one tag at a time then here is the easiest way I can think of.
First, back up your library for in case something goes wrong.
- Find all books with the tag (name) of interest using search, e.g., tag:"=name"
Then assuming there is more than one book found:
- Select all books (Ctrl-A on Windows)
- Press E for bulk metadata edit.
- Enter the tag (name) in "Tags to remove" on the basic metadata screen.
- Enter the tag (name) in the #characters box on the custom metadata screen.
- Press OK
Voila. The tag (name) will be removed from tags and added to #characters. Carefully check the results and restore the backup if something went bonkers.
If only one book was found then do it by hand with single metadata edit.
It is possible to do all of this in two S&R operations but it requires some template-fu that is hard to explain and easy to get wrong.