If you could temporaly add a new vmfs volume, you could clone your VMs on it. Depending of what hardware you have, you could then remove the volume without disrupting the data on it. It can be later on, reassociated to your ESX and then reinventory each VM.
That would be one quick form of safety backup.
When doing an upgrade, I check mostly for elements that would not have been tested properly by VMWare. One exemple, in our installation, the local group Administrators has no rights in Virtual Center. Because I am not using a default configuration, I will re-authorize this local group before upgrading Virtual Center.
I am using DRS and HA, so I will look for information about coexistance between 3.01 and 3.5 within the same cluster. For exemple, a VM with the VMtools v3.5, how does it perform on ESX 3.01. Things like that are not mentionned in the upgrade guide. You will notice that there are more text about upgrading from 2.5 so it kind of incomplete.
Another example is upgrading to solve a problem. That problem could create another one while upgrading. So the closer you are to a clean default configuration, the best chance you have that it will work fine.
I am currently running ESX 3.01 on a couple of servers and because they work fine, the only reason I will upgrade is to keep up with the new versions. But I will do that only around april. I like to wait for the first few fixes. If nothings major, then it is a good to time to move. Meanwhile, I watch this forum and prepare my todo list.
