ID3v2 Tag Editor is a work in progress. It supports converting from v2 to v1 and vise-verse, but it doesn't have a batch mode that I can find. It might be worth a shot though.

http://www.audioxl.com/id3v2/index.html

I was thinking along the same lines, and am currently hunting for one. If anyone knows one, PC or Mac, let me know. I have found
one for the Mac (MP3 Inspector) that is still in development, is well-thought-out, and plans to support copying tags between formats
in batch mode, but has yet to actually add that feature.