I booked this a couple of weeks ago on IAP. Had to make a change and Delta would let me go through until the very end (after credit card number entered) and then return a "fare no longer available" message. But the fare is available. So, I gather that despite the tight alliance between DL and AmEx, IAP booking needs a call to AmEx for changes. DL asked for $4 fare difference, and it ended up $260 with AmEx.