How can I downgrade the OS on a Mac to the original one it came with?
You can reinstall the OS back to the same version that it shipped with by using the restore discs that came with your Mac to wipe your disk and reinstall Mac OS X.

However, just because Apple has officially dropped support for XP and Vista with Boot Camp doesn't mean it will stop working - the most likely situation will be that new computers will have hardware that Apple will only provide Windows 7 drivers for. Existing hardware should be unaffected.