True, you would not want to back up anymore with the same Time Machine drive once you upgrade to Mavericks.Just create a backup on an external drive. If something goes wrong you can just revert back to what you previously had. You can also keep a Time Machine backup but I'm not sure how well that will work if you continue to backup when using Mavericks.
If you create a bootable image using a program like SuperDuper it will be the exact same as if you were using your system before the upgrade.
But, you can't simply downgrade from Mavericks. You'll have to reformat and restore from a backup.