“Medium: Same as easy but go from the derivative to the base.”
I can’t quite recall, but I think I did exactly that with Ubuntu -> Debian once upon a time. I think Ubuntu was only a year or so old though, so there wasn’t a huge amount of divergence back then. As a bonus anecdote I also attempted a semi-successful build of Gentoo on a PPC Mac around the same time (nothing before or after that has compared in its level of nightmare).
I had forgotten about doing that myself. I did that on a couple servers once the distros had full 64 bit builds. Does that technically count as an architecture swap in-place as well?