As mentioned above, it would be the SkyTeam upgrade program. Unfortunately the details were removed some time ago from the skyteam website, so applicable fare classes are no longer listed... however it used to be a full fare Y (or close to it depending on the airline).

You could call DL to inquire how the SkyTeam upgrade program works and what fare is required and how it would have to be ticketed... but expect it to be a difficult process and a very expensive ticket.