Walmart charges when products ship like a normal retailer. The posts about unwanted charges from them here came from unwanted authorization charges hitting debit cards repeatedly and causing issues that way. I've had three authorization charges of $90+ hit my card but they reverse themselves away a week later.
But yeah, if you've got a looming expiry date situation you're going to need to update that card with them at some point.
I spoke to Wal-Mart this morning and it's going to be a mess. You can't change your card through their website, they aren't allowed to take anyone's card and change it for them through their customer service and you can't go in store to get it changed in person.
My only option is to place the order again once my new CC comes in, which will be right in the middle of a couple of my orders this fall. Not only that, but even though the rep put in a note to honor the discount, I have a feeling that won't be so easy. Pretty sure that at this point I can say goodbye to a couple of E3 discounts.
This is why you order with Amazon, they're shipping might be garbage but issues like this can be resolved within minutes and without even speaking to anyone.