But it's not so much different either. I've ben using this "two primary root partition"-setup for quite some time on all my systems at work. Although I usually prefer to reinstall my desktop systems, this is how I would handle the "revertable upgrade" case:
- create an new filesystem with a different label on the unused partition
- copy the old system over
- change the label for the root filesystem in /etc/fstab to the new one (I would also grep through /etc/ for the old label eg. xubuntu1204, debian60, just in case)
- chroot into the new system and install grub to the new root partition and update-grub (or install and reconfigure extlinux... :)
- activate the new and deactivate the old partition
If anything goes wrong just reboot into the old root. Using "MBR" this is a simple matter of pressing the shift-key and choosing the right partition at boot time.