Go to Payment and select Cancel Subscription. What this will do is remove your credit card from their records, yet you will be able to continue playing until the end of your purchased time. It simply prevents them from ever charging your credit card for a subscription renewal.
At any time between now and when your 30 days is up, go to Add Game Card; the game will not begin billing the game card time until after your current prepaid time is complete.
Example: If you have 15 days left before your subscription needs to be renewed and you add a 60 day game card today, you will have 75 days of game time available.