Hi Lyle,
It is true if you are not using the Same DB. Because if you are installing a fresh 5.1 using the same DB, a rollback to your old vCenter with this DB should not work after an upgrade of it.
But if you are using a new DB too, you will loose all your vCenter's parameters.
So my guess, the best way, is this step :
- Shutdown all your vCenter Services
- Backup your DB
- Backup your vCenter
- Start all your vCenter Service
- Upgrade the vCenter.
But in case of failure, you should be able to rollback and you will preserve all your configuration.
Regards,
Julien.