Cotton - I think it would be better/cheaper to invest in a second bike, plus you may have chain length problems switching between different rear cogs depending how different the cogs are. You would need a chain big enough for the largest cog on the biggest set. A second set of wheels, tires, and cogs is expensive and a hassle to always switch in/out. You could probably pick up a used Hybrid or road bike fairly inexpensive. A hybrid or road bike will move you into 700c wheels that are faster than mtb 26 inch wheels with slicks. My 2 cents.