Cassette change will run you $40-50 for cassette and tool but if you have a lot of miles on the current setup you -may- have a chainwear/cassette incompatibility.
This tends to pop up in the 3-5kmi range. I generally get 2.5 chains per cassette. I suspect your road miles are going to be a lot more than your crit miles so it will not be simple but you really do need two different cassettes/wheels at your level of riding. That 27t cog will make you spin. Steve