Depends because the partition and directory structure has changed over the years, but it generally boils down to rsync -avP6AHXx --numeric-ids src dest.

It requires some fiddling afterwards (not only grub-install: adjusting fstab UUIDs, adapt network interface changes, etc) but it's generally pretty painless.

I almost posted earlier about the after-adjustments. I have 5 and 8-year installs, but I've never bothered to migrate across machines thinking it'd be too much of a hassle.

Ah thanks - my go to rsync flags are -hvrtP which seems like it's quite similar. Still curious about what you rsync? Presumably this is within chroot during installation?