Generally, going up or down 3 to 4 teeth on chainring is equal to one tooth change on cog...also, generally, one tooth diff in cog equals about 5 rpm pedal cadence at given speed. Unless you are at absolute limit of track ends/dropouts you could reasonably expect to bump cog down one to two teeth with no issues other than a somewhat longer wheelbase. Also, a 2 tooth diff in cog will feel huge so prob best if you just went down one tooth and see how you like it.