Question: are 8 and 9 speed cassettes interchangeable?
Answer: Simply put - 8, 9, 10 speed cassettes all fit on the same hub. A seven speed cassette will fit on an 8 speed freehub with the use of a spacer. (One notable exception is the Dura Ace FH-7801 hub with an alloy freehub which will only accept 10 speed Shimano cassettes - the newer Dura Ace hubs can run 8/9/10).