My apologies for the delay in response.
To change the credit card associated with your subscription you would need to follow these steps:
1. De-activate your current subscription (you can do so via your Subscriptions page at http://twittercounter.com/pages/subscriptions).
2. Wait until your subscription expires (one month after the last payment, or in your case on the 17th of January).
3. Re-subscribe with your new credit card using our signup form at http://twittercounter.com/pages/premium.
Note that any data we have gathered for your Premium subscription will not be lost when you de-activate. Once you've re-subscribed you'll automatically have access to it again.
Could you please send me an email regarding something wrong with your stats? The email address is firstname.lastname@example.org.