Just FYI - That defect only impacted systems which boot from ZFS - mine did not.

The installer would stop an upgrade if it found a mounted volume - it apparently checks for zfs volumes as follows: sudo zpool list -H

I could get around the installer's ZFS check by unmounting the drive: sudo zpool export poolname

I upgraded.

Then remounted the volume: sudo zpool import poolname

Then I was up and running...