JiveTurkey

Actually, if you keep the same derailleur on, you don't have to shorten the chain. Your drivetrain won't know the difference, it'll all be the same except it will just think you're not using the biggest few cogs. That being said, I'd shorten it anyway because there's no need for the extra length. Edit: Like SweetLou said, you should get a new chain if the cassette has a decent number of miles on it, otherwise the new cassette will wear faster to match the stretched pitch of the old chain. You can keep the old cassette and chain as a backup or sell them together for a few bucks. Oh yeah, and most any 9-speed chain will work, but SRAM will say their cassettes and chains work best together.