The discount outlined in the original emails sent last November to registered beta testers of 2.0 involves purchasing a new subscription. If you are an existing subscriber, you may want to look for a retail deal on OneCare before renewal and use the retail key to renew at http://billing.microsoft.com - signing in with your subscription LiveID and using the renew link and the option to use a PIN or token to renew.
If you want to purchase the beta discount offer, you need to refer to the original email for the link and purchase a new subscription using a different LiveID than your current subscription and *then* be sure to cancel your existing subscription before renewal by contacting support.
-steve