I do what kdog listed. In year 1 I split my practice time into roughly 2/3 to the new offense, and 1/3 to the old. And I play the old offense, except maybe in a few blowout games I'll mix in some new plays in the 2nd half. 2nd year practice the new only, but start mixing in the new offense in games vs sims. Then in year 3 the new is ready to go.