You should be notified by Microsoft before the end of your subscription, but even if you are not, when it expires, simply uninstall OneCare -
http://social.microsoft.com/Forums/en-US/onecaregeneral/thread/732e6371-2e71-479a-8cd6-b189941ed4fbAnd then download and install Microsoft Security Essentials. It is free and available now. No need to switch until your current subscription expires.
http://www.microsoft.com/security_essentials/-steve
~ Microsoft MVP Windows Live ~ Windows Live OneCare| Live Mesh|MS Security Essentials Forums Moderator ~