Here is my understanding of the DL policy...

1) you can buy up to any full Y or FC fare (including Q-UP)
2) the change is allowed only on the same flights as the original itinerary - not just the same routing, but the same actual flights
3) although the new ticket has the fare basis and code of the upgraded fare (eg. the new ticket will show "A" class and the new fare is the published A fare), I believe there is some endorsement to carry over the original fare's restrictions. However, Delta A fares are generally non-refundable and carry the same change fees anyway, so the endorsement is not always necessary

I would be satisfied if the restriction was any fare-->Y or any fare-->R/A/C/D. I'd love to see it rolled out as any sub-V fare to any V+ fare, but that's probably asking for a stretch. For simplicity, if they agreed to do it, it would probably mirror the Delta policy, which is upgrade to refundable Y or any FC fare only.