Wtih a ringed engine, and using fuels with 15% or less nitro, you can pretty much run whatevery you want whenever. You'll have to reset the carb a bit to accomodate the fuel change, but not much else. As long as you have enough oil in the fuel, you'll be OK.

If you make a large change, such as from 10% nitro to 30% nitro, then you'll have to do a bit of breakin, because the engine will run a lot hotter and the parts fit will alter drastically at the hotter temps.