It is highly illegal, you cannot set back your odometer, because your just changing the engine, what if different things go on the car like brakes after 2,000 miles and the odometer says 12,000. Things like that should make you say, hmm..ya know I dont know what im talking about. Because, if you reset the odometer on any car, it is illegal everywhere.
There is no law that says "You can lower your odometer when you get a new engine." but there is one that says "Odometer Rollback is illegal". I think it's even punishable by jail time.