You can manually change tag types by typing character:tag, coypright:tag, artist:tag, depending on what you need it to be.
For some of the larger tags, however, this can take awhile to change.
Posted under General
Any given tag can only have one type. So you can't have separate artist Mello and character Mello tags. While that would be a useful feature for power users if implemented alongside a mechanism for specifying type at time of tag addition, it would create difficulty in tagging for most other users - if you just typed "mello" for the tag, the system wouldn't know which you meant unless you clarified, and it could get really tedious to specify a tagtype every time you used a tag.