with the correct adapter. Most brake manufactures make adapters that go with the brakes and are dependant on the size of the rotor. I.E. buy 160mm BB7's and want to go to 203's later? Buy the correct front and rear adapters and the rotors and you're in business. There's nothing about the rear of the FSR XC that would prevent you from using 203mm rotors. However, you should check with Specialized and see if seat stay is rated for that big of a rotor, you should also check with your forks manufacturer and see how big a rotor it is rated for as well. Most 30mm QR forks are only rated up to 180mm rotors, some only to 160mm. So check out the manufacturers recommendations before you pull the trigger on 203's.