Business as Usual
To be honest, I’m surprised that they haven’t already. Intel has had a good innings with Apple but, up to now, Apple seems to switch every ten years or so (Intel is now at 13 I think).
The Mac switched smoothly from 68k to PowerPC, and from PowerPC to Intel without a hitch (and that’s ignoring all of the other architectures that it passed through in prototype form, or only incidentally (Sparc, PA-RISC and so forth)
There’s always a small speed bump, but no one seems to notice or care - and ARM is definitely big enough and fast enough (at least, in Apple’s versions) to take the strain. Just imagine what Apple could do with ARM of the space and power constraints of having to run in a phone or tablet were removed…