I'd go the extra mile and create an archived backup (perhaps compressed with 7zip / p7zip, using maximum compression -mx9 parameter) of the .vdi / .vdk or whatever disk image file you're using, before making the upgrade.
Exporting the machine in its current state as an ovf "appliance" and storing it on another (perhaps external) hard drive is also a good security feature.
But the above is just adding layers of redundancy. "just in case"
