Interchangeability, no idea. The RR-engined Mk.IX Spitfire was a Mk.XVI with a Packard Merlin but whether that was for information or it really mattered I haven't a clue.
At least one UK Spit Tr.IX is flying with a Packard which I suppose technically should make it a Tr.XVI! It was one of the Salt River scrapyard airframes and has a new build fuselage; I can't imagine there would be much difference between mounting a RR Merlin or a Packard example in a Spit, but then again...