There’s certainly a lot that’s similar, and in most cases the boards can be swapped directly. However, with essentially two versions to the NAC72, there may be an issue (possibly with the NA729s?). I don’t know for sure where, but I’m sure @NeilS can enlighten us.
Yes. The first version of the 72 has 2 pair of BNC input - one marked Phono and the other marked Aux (which usually had the 328 V variable line level boards fitted). The 2nd version has just one set of BNC’s - for Phono. There is another DIN input instead of the 2nd set of BNC’s - which is now ‘direct’ (no boards involved) - marked CD (!!).
With the NAC32 and NAC72, pretty much all the circuitry is on daughter boards, so it is possible to just swap out old boards for newly serviced ones. Of course that won’t fix any other issues there may be with things like the pots, switches or socketry.
To answer the OP’s original question, yes you can use the 72 boards in a 32.
The 729 was used in the 72 & the 329 the 32 - same PCB, but the 329 has a couple more components fitted to facilitate the mono/stereo function.
So the 32’s mono/stereo switch will have no function with 729s fitted.
Easy way to tell them apart:
729 = 4 gold receptacles.
329 = 5 gold receptacles.