Well, you see, it would actually be about twice as difficult, because -
oh you just don't know how game development works okay.
Don't want to get into an argument, but if every song is basically just a file with some audio, animations, and some beat-matching midi stuff, couldn't they just write a batch conversion tool to convert everything to their new format?
I'm not saying they should throw up 3,000 songs on the marketplace day 1, but it can't actually be that difficult to convert the files from one format to another.