You can upgrade your marketplace subscription from standard to premium at any time.
Before you can upgrade your subscription to premium, you must first cancel your existing standard subscription using the following steps: First, login to your account and select the “My Account” link in the top left. Second, once on your account page, select the “My Subscriptions” tab and then choose the “View Orders” link next to the marketplace subscription that you wish to cancel. Third, select the “Cancel” link to your most recent order and confirm your desire to cancel.
After your standard subscription is cancelled, you can then purchase a premium subscription here.