Just thought I would relay this, since I don't recall seeing it mentioned. (Maybe it's just me, but I think it's a problem with Delta's software.)

I had a schedule change on a flight ATL-DCA-ATL, and so was able to change my return to an earlier time at no charge. Everything worked fine until the final button, which returned me to the last page with a warning in red that my credit card number was invalid. This, despite the fact that the charge was $0.00. I tried switching to another card to no avail. I tried the app; same thing happened. Finally called in. The same thing happened with the agent. She laughed at the ridiculousness of it. She had to call another department for help, and then call me back. I mentioned to her that this was partly paid for by gift card, since I had a hunch that might be causing this problem. When she called back, I asked her, and she said, yes, she was told that was probably the culprit.

I feel like I must have done this successfully before, since I've made numerous bookings using gift cards to utilize the Amex Platinum credit, so I think it's a recent thing.