I would like to upgrade my current US/Canada Subscription to Premium - which currently has a 50% discount if you buy a full year. The Premium offering is a great deal because it includes a subscription for US/Canada. However, I've already purchased a one-year US/Canada subscription and it expires in Sept 2012. What happens to my existing subscription if I purchase the Premium Full year? Does the previously paid money get applied and I just pay the difference? Or does it extend my US/Canada Subscription?