Most changes you’d never even notice even being a nut about the P51. Lots of changes in the control mechanism, electronics and aviation instrumentation. Many also change the aerofoil profiles(ailerons, elevator, rudder, flaps, etc) to provide more control surface and lift for more radical maneuvering. Some change the canopy section to more modern polycarbonate parts for more visibility. This is all on top of modifications to the Packard engines which already put out in excess of 1600-1700HP.