You can only configure Centralized Backup if the destination device is a hard drive or network Share formatted to NTFS. If you are planning to use either as the destination and you've configured backup for a single PC and now wish to change it, try the following:
Open OneCare
Click Change Settings
backup tab
Click Configure Backup
Click change settings in the lower left
Click your PC on the left.
Uncheck the selection for "Turn on backup for this PC."
Click Next and save the revised plan.
Go back to the backup tab, configure backup and click Change Settings in the lower left once again.
Configure backup as desired.
Save the revised plan with the new selection.
-steve