If you have a diablo tuner you can do this. Check out a tire size calculator Tire Size Calculator - Compare Tire Sizes to see what the difference is. You could go a bit before doing it, it is not crucial to do it right away.
If you use the tire rack website you can calculate the OP of each tire. If the OD is closer no need to change. I went from 18 to 20 on my 2010 and it was less then 1 mph difference. At most your 2 mph off