Just noticed that there are actually only eight sets you have to complete on Medium. I was singing Suffragette City on Hard and thinking "Wow, this isn't so easy. I don't remember singing this before." Looked back at Medium, and sure enough, here's this new set (Suffragette City, Highway Star, Tom Sawyer, Run to the Hills, Foreplay/Long Time).
Nice touch. You don't need to finish them to complete it on Medium, but you get to go back and do them once you've unlocked them on Hard.