I'll try to keep this short. Six month old Trek Rail 5 has SRAM 12 speed 11-50 cassette with a Shimano 8 9 10 free hub (according to the tech specs on site). SRAM SX derailleur and shifter. New chain is skipping like crazy. Called bike shop and new cassette is most likely needed. I only use maybe 6 or 7 of the gears. Can I get an 8, 9 or 10 speed cassette, maybe Shimano, and use current derailleur if I switch the shifter? Would I need to buy another chain also? Thanks for any help. I really don't understand drive train compatibility yet.