Well I at least hope that the change is one directional; in that you can drop the difficulty, but not raise it again ~or... if you can, then it intentionally doesn't count, and the game doesn't record that you played the higher difficulty if you've dropped it during the game ~even if you raise it again later. IMO mixed difficulty settings are a cop-out; especially if the game evaluates/ or adjusts its content based on the difficulty settings.