Apple seems to be getting better at this: The transition from 68k to PPC was a nightmare. The transition from PPC to x86 was surprisingly smooth.
I bet the next one will be close to seamless.
They had already had NEXTSTEP running on 68k, SPARC (I think) and i386 before Apple bought NeXT. Architecture independence has been baked in all along.
HP-RISC also. The old compatibility guide was kinda big to get faxed to you.