Hi,
We've upgraded a couple of times so I've some experience.
1. Not a big change. I think we had some problems couple of times and the installation failed but even those failures didn't break the DUAS installation. I don't remember what problems we had.
2. I think the upgrade process takes a backup and restores that if there was a failure.
3. We did this also but we didn't have to restore the files so I don't know if it works.
4. My understanding is that a proper rollback is automatic in the upgrade process. If the upgrade fails, it will restore the backup itself.
You can also back up all the items in UVC before the upgrade. Make a package(s) of all uprocs, sessions, tasks etc. which you can restore if a worst case scenario happens that you have re-install the whole DUAS.
When you have everything backed before upgrading, there's no real risk.
Br,
Aukusti