The disc would have a database of mis-genred songs. When it's time to display the genre of a song, the song's original genre is ignored and the new genre from the database is used.
I think this could also work for tiers (please get Constant Motion into the devil tier for guitar)
Gaga 01: "Pop/Dance/Electronic" did not exist at the time, did it?
"Other" songs: I bought SpongeBob for goals and the Bee Gees for no reason, and it amused me when I got a "Random Other Setlist" in the Road Challenges.