In reply to Lynsety:
Generally, yes its fine, although discs vary in performance a bit, so you may find another manufacturer's discs don't work as well due to different cooling rates.
Also, if looking at Floating rotors (the pretty 2/3 piece rotors) you might need to be wary of clearance around the Calliper.
Whilst you are doing it, consider if you want to go up to a 180 at the front (do you get brake fade, are you planning a trip to the alps etc?) Probably fine, but depends on the riding you do