i have an old 7 speed derail on a 9 speed bike, works fine, technically they are not compatible (range issue) but i've never had a problem, most of the time you can get away with it but its not "right" it was my understanding that 8speed cassettes are slightly narrower (distance from big cog to little cog) than 9 speed ones but i've been told by people on this forum that that is not true, regardless it seems to work just fine most of the time (but you can't be 100% sure w/o trying