In case anyone wonders how this played out. At the end of the day, I shutdown the host with the running workloads on it, kept it at BIOS boot so the 8.3 installer would upgrade it cleanly, and then booted it back up. For reasons related to my particular setup, the down time was more than 60 minutes. But that's not related to XCP-ng. The XCP-ng installer was maybe 10 of that. 20 of that was 3 reboots on a server that takes ages to boot.
One thing that is a bit difficult is that the 8.3 installer doesn't notice ZFS volumes or help with ZFS. So after the upgrade, I have to follow a modified set of these instructions (modified because the package names have changed over time) to install ZFS and its components into the system. So the sequence is:
Shutdown
Install via ISO
Reboot to single user mode
Run yum update to pull in all the updates
Run the ZFS commands to install ZFS on the host
Reboot again
Run normally
Of course, I ended up rebooting more than just those 2 times, because I forgot that I would need to install ZFS (even though I did it on 2 different hosts already )
So now the pool is fully upgraded, everybody is happy. I'm now working on creating shared storage across the nodes. It's amateur hour over here. I appreciate all the help.
