Have you gotten an e-mail about your payment method? If not, there's a chance you'll get such an e-mail when they receive your item. At which time you would be able to offer an alternative payment method. Do note though, that the window to do so will be small. And never cancel a pre-order, no matter what the associate over-the-phone says. I learned the hard way that it's never the correct answer to an order issue. If you're told that, the person telling you is wrong, ask to speak to someone else or hang up and call back.
It is odd that Walmart doesn't have a simple "update order info" option for online orders, though. In 2015 that should really be standard for all online transactions, especially pre-orders.