hi all - i have a question on cassettes. right now i'm riding an approx. year old 11-27 dura-ace cassette. my question is: since it comes in a number of pieces, can i swap out my smallest gears (the last two, which come as one piece) and move down to a 28 without replacing the entire cassette? i'm on a serious budget (grad student), and need something a little smaller at times for some lighter work outs. can't afford to spend $200+ on a whole new cassette. it seems very doable, but i don't know where i can purchase a subset of the cassette. any ideas???? thanks!