Originally Posted by
xliioper
The cabin is identified by the second to last letter in the fare basis code. If it's a "D", it's a D1 fare -- end of story. DL doesn't play coach with instant upgrade games (that's an AA thing). The X at the beginning of fare basis code just means there's a dual inventory check. Most domestic FC fares are like this and no one has reported issues with getting credit for those.
DL does have active "sell-up" fares for Premium Select; it would not surprise me if they now have them for D1. But I agree, crediting these should be fine.