Being a Flying Blue (AF/KLM) member (Platinum / Gold / Silver) puts you in the upgrade list on DL flights. It's usually meaningless because you are at the bottom of the upgrade priority list after all DL's own frequent flyer members, however in theory you can be upgraded. I was only 1 away from the last person to be upgraded on my short hop from BOS to JFK last week so very nearly made it. I think I have only been upgraded once in the last few years on DL due to my FB status.