A727 Torqueflite questions (for gurus only lol)

U need a 76 or later tranny to be able to use newer planetaries

I'm working on a '78. Keeping in mind that spline angles have changed...... the question in my head is; if the rear planetary ring gear stays with the correct output shaft, then can only the planetary itself be changed ( since it only needs to match up to the rear drum)?? Or can those parts not mix-n-match?? Not talking about the front planetary here at all.
TIA, Lefty71