I suggest making a disk-to-image backup of your whole boot drive using clonezilla, to a second external disk.

To test recovery, take a third spare drive, restore to it from the backup image using clonezilla, and replace your boot drive with it. If this boots up well, you could even do your upgrade tests using this spare disk, restoring from the backup file as necessary. Then once you're confident, put your boot drive back in and proceed to upgrade it.