Your Premium Seller membership begins on the day that you upgrade your account. You'll earn the Premium Seller rate on all sales that occur from the day that you upgrade your account, until the day your membership ends (if not renewed). The Premium Seller rate cannot be applied retroactively.
If you decide to upgrade to a Premium Seller account, you'll set up and pay for your annual subscription via PayPal. Once you're all set in PayPal, your Premium Membership subscription will automatically renew each year.
If at any point you'd like to cancel a renewal or change the card associated with your subscription, you can make those adjustments from within your PayPal account.