You did not need to reinstall OneCare, you simply needed to renew with the key from your new retail copy. When you activated the new install with the same LiveID as you existing subscription, it inherited the subcription status.
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.
-steve