Just wanted to leave this here in case someone else ran into the same problem. Maybe it's covered somewhere already, but I had no luck finding any information about it. Anyhow, I had a 12.3 system which had a UFS boot/root and a ZFS /usr. I began the upgrade process using freebsd-update following the instructions from the handbook. But, after the first reboot (into the new kernel) /usr failed to mount and I was greeted with an error about not being able to find /usr/libexec/getty instead of a login prompt. So I booted into single user mode to see what was happening. Running "zpool status" showed no pools. So I ran "zpool import" and saw a message about the pool being last used by a different system and advice to run the import with the -f option and the pool name. I ran that and the pool was imported successfully. I then mounted the filesystem manually and saw all my data was there. Great. I rebooted again, but was again greeted with the error about not finding getty. Booted into single user mode again and "zpool status" was again showing that thepool was not imported. I ran "zpool import" and the pool was imported and I could manually mount it but upon rebooting I was stuck at the same spot as before. So I once again rebooted into single user mode, imported the pool, and mounted it manually. I then ran "swapon -a" and exited to multi user mode. At that point I ran "freebsd-update install" again and let it do its thing. I rebooted after it was finished and it booted correctly that time and I was able to finish the upgrade as usual.
