A clean install will be… cleanest… but it turned out that my unstable install was only about ten packages away from Arch stable anyway. The weird behaviour I initially found was down to files from filesystem being removed and left unconfigured when reinstalled during the migration. Now I’m running my own personal Arch derivative (I overlay a few packages) and I think I’ve found fewer issues than with unstable over the same period (while finding/fixing a few upstream ones too).
Oh, also, if you need older LTS kernels, I run this too: Unofficial repo for older LTS kernels