With all the things MP3Tag supports, I would have thought it would allow the copying of Lyrics3 tags from ID3V1 to ID3V2 UNSYNCHEDLYRICS field, but I can't seem to find anything that does this.
I have a lot of stuff with the the Lyrics3 tags, and it's a real pain to transfer all the tags by hand.
In the menu bar, select Actions>Actions (Quick)>Format Value
You can select any existing tag as the source and use that to redefine another tag this way.