Reinstall openHABian with bullseye from scratch will be the lest intrusive method that is guaranteed to work.
It’s more involved than that. You’ll need to manually edit the apt sources files first to point at bullseye instead of buster.
I usually try this first because many of my RPis are hard to get at. It works roughly 75% of the time. If you’ve not plugged in to a keyboard or monitor make sure to use screen or some other command to keep the upgrade running if you lose your SSH connection or else you are hosed.
I believe that’s what is meant by reinstall openHABian from scratch. Just make sure the new version of OH is the same as the old version. It usually doesn’t work to install a backup from an older version of OH to a newer version of OH. So this approach is the recommended approach.
The only gotchas are the openhab-cli backup only backs up openHAB’s configs. If you’ve edited configs for other openHABian services (e.g. Mosquitto, NodeRed, etc.) or data (e.g. InfluxDB, etc.) you’ll need to figure out how to backup and restore those yourself.