The franchise has been out for, what, five years now? More? The move up to hard is the most difficult transition in the game, because it requires you to actually move your hand on the, uh, fret board. If you got to One on Medium, you can probably do some of the really easy songs on Hard. Once you've got that under your belt, it's just a matter of time. You'll get there eventually with no special effort required on your part. Bass is easier than guitar, too, so that's an easy way to make the transition.

Also, once you've moved up a difficulty level, it's impossible to go back down.