As I was heading out to take the dogs for a walk; I came up with another idea. IF you make sure that you have a Shimano "GS" rear derailluer, it definitely will work with a mountain cassette. If I remember correctly, the IRD cassette jumps from a 28T to a 32T which for some people is too big of a jump. The other option is to use a SRAM 10 speed mountain cassette which is a bit more even in the spacing on the cassette, but very expensive. ($300)

I love thinking about all of this sort of thing, thanks for sharing