AFAIK the old card should be still valid for the whole month so if you are not pressed wait till the sale is over.
Here is my question to steam support and their answer.
Hello
Do I need to update the expiration date of my stored credit card now that my new one came? Because I can't find a way to do it except deleting my card info and reentering it with the updated expiration date. If I delete my card and reenter it with the updated expiration date will I get a 1 week shop ban?
Regards
Support answer:
Hello
Thank you for contacting Steam Support.
You can update your payment information during the purchase process.
Otherwise, to remove your credit card details from your Steam account:
Log in to your Steam account.
In the upper right hand corner, click the "[Your username]'s account" and select "Account Details" from the drop down menu.
Under "Your Steam Account" on the right-hand side you will see your card type and the last 4 digits of the card that was saved. Click the "Delete" link, to remove this information from your Steam account.
NOTE: If you do not see a "Your Steam Account" section to the right of "Licenses and subscriptions", this means that your credit card information is not stored.
The next time you complete a purchase, you will need to enter your credit card information. If you do not want your credit card information saved, uncheck the box that says "Save my payment information so checkout is easy next time".
All payment information is protected by industry-standard RSA public key encryption and is only stored in our system for the duration of your transaction.
If you have any questions about our privacy policy or the security of your information, please review our Privacy Policy: Steam Privacy Policy
http://www.valvesoftware.com/privacy.htm
Please let us know if you have any questions, or if you encounter any further difficulty.