You cannot extend your subscription with the retail key at this time. What you can do uninstall the current copy that you have and abandon that subscription. Then install OneCare again (use the CD or install the trial from
http://onecare.live.com ) and then subscribe with a new LiveID, creating a new subscription, using the option to activate with a retail key. I expect that new subscriptions will be completely disallowed after Microsoft Security Essentials is released later this year.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator