Finally got around to it. Thanks to some tips from a fellow Linode/NetBSDer I could avoid almost all of the pitfalls:
- Ahead of migration edit
/etc/fstaband change all
wd0.*. They were "1"s because XEN required a boot disk, but since that was going to be deleted with KVM I knew the disk would change to "0".
- Also before migration edit
- And ideally before migration, but I forgot and only did after migration: Edit
And that's it. Was much less painful than I was anticipating. At the moment I don't have serial bootblocks so I don't have lish access, only glish, but come the next NetBSD update I'll correct that.