I bought a used Trek 4300 for a song and dance to get into upgrading, and tooling around with the bike; more or less to learn about hjow to work on my other bikes etc. My question is: its a 21 speed with entry level Shimano shifters derailleurs etc. I have no particular desire to upgrade to a 9 speed cassette, and the guy at my LBS said this really isn't possible anyway: the cassette wouldn't fit. He also said that 7 speed cassettes are practically extinct, and new cassettes with 7 speeds are junk. I consider myself an intermediate rider, with no desire to ever become a serious mountain biker. Are his remarks geared toward high end mountain bikers as opposed to a hack like me? Should I worry that soon I won't even be able to find a 7 speed cassete, and a non-9 speed chain to go with it? Just wondering, before I spend too much time playing around with this trek. Thanks!