The id3v2 tool beta I am using internally does *almost* what you want..
While separating the v2 tags from the music content, it appends a v1 tag
to the music if one did not exist already. It would be trivial to add a flag
to force it to always create/replace a v1 tag based on the v2 info.

As for prompting.. life gets messier.

My best bet there would be to code up an awk script which
pulls out the v1tag (my "mp3sort" script already does this),
and uses "id3v2" to pull out the v2tag. Then a few lines
to compare & prompt the user .. presto.

email me if you want it -- maybe quote this msg and your original
as a reminder of what the heck we were discussing.

Cheers