I think no one can tell 100% sure that it will work… in my case, with a NUC, I’ve been having some bumps.
My suggestion: configure the Google Drive Backup addin for having automatic regular backups, and make sure you have a recent backup out of your server. So if things go bad, you can always reinstall a fresh working version and restore from your latest image (that’s exactly what I’m doing now).
It’s not an option for me to backup to the cloud.
I have seen that you can upload a backup from the filesystem but how to download it from the HA system?
If the upgrade goes wrong, will a fresh install work anyway?
If your upgrade goes wrong the best restore is indeed a full clean installation then reload the last full backup.
As to how you get it back? Ftp, SMB file transfer? Really you just need to get it back to whatever you’re running on.
That said. It’s a shame cloud isn’t an option for you it really is the easiest. I backup every night to the cloud and if anything ever goes wrong, I reinstall HA from current image, fire it up, create a dummy account to get through the welcome, install the Google drive backup addin and restore latest. The whole process takes about 30 minutes.
In your case you’ll install HA, get through first run experience, do something to enable file transfer in, copy the file to your HA hardware then restore.
Your takeaway from this needs to be, whether you are ready to upgrade or not, definitely get your backup setup and make sure you are copying it off box. It can be a lifesaver. It also makes it less worrisome to upgrade.