Should I swap '72 sbp discs to '73 bbp discs?

None that I can think of. Your proportioning valve may not be setup perfectly for the 73+ disks, but a minor issue since the optimal setting varies with tires, pads, weight loading, etc anyway.

Why do you want to swap? The early Kelsey-Hayes was probably a better design than the later sliding single-piston calipers. The main reasons I can think to swap is better parts availability and if you want BBP wheels. Of course, best to swap to a BBP rear also so you don't have to carry 2 spare tires.