Better yet - pick up the retail copy for a discount whenever you see a good price and renew your current subscription at any time with the retail key. No need to uninstall/reinstall. Your subscription expiration will simply be extended by 12 months. You can do this at any time.
Go to http://billing.microsoft.com and sign in with your subscription LiveID. (Open OneCare and click help/about to view the subscription LiveID)
Select the OneCare subscription and click on the renew link.
Choose the option to renew with a prepaid PIN or token which is the retail product key.
Enter the 25 character key and proceed.
It can take up to 24 hours for the subscription status to be updated in OneCare if you are renewing within 30 days of expiration and 5 days if renewal was performed between 90 and 30 days before expiration.
-steve
