Answer
STAFF
Sam Wierema
31-Aug-2011 06:57:23
To change the credit card associated with your subscription you would need to cancel your current subscription (you can do so on your Subscriptions page at http://twittercounter.com/pages/subscriptions).
When your Premium account expires (in your case on the 13th of September) you can re-subscribe via the signup page at http://twittercounter.com/pages/premium, opting to pay with a new credit card.