To get back on topic...

A change like this isn't going to be easy.
You essentially need a new operating system and that's a huge job.
The question is how can this be done without taking years.

I guess the only sensible way to keep backwards compatibility is to create a PowerPC emulator of some form.
I worked fine for Apple but theirs was created externally.

Then it gets really interesting - and complicated.
How much of the Amiga/MorphOS API can be used?
Can libraries be used to hide the lower level changes.
Can you create an OS that enables you to port existing apps without too much pain?

More importantly can you use other existing work from other open source OSs - even if it's just a stop gap.

Haiku started with the NewOS kernel and components from BeOS. The BeOS parts have long since been replaced but it meant they could get a functioning system quicker.

OTOH they have also suffered feature creep to the point that they've never done a full release - don't fall into that trap!

Then there is the thorny problem of which architecture to port to. If you go ARM which version?
Hint: Go 64 bit ...it'll save a load of hassle, though I suspect you know this ...and look into LLVM/Clang.

Of course once you're up and running you'll have en entirely new problem: Expect lots of processor cores, but don't expect them all to be the same.