I thought that AF/KL take control of the flights at the 48 hour mark. At least I recall DL saying they can no longer change seats at that point. As previous poster noted, C+ is not a separate fare class on TATL routes and so there are no auto-upgrades. A Gold would likely need to choose after the window opens at the 72-hour mark and before AF takes control at the 48-hour mark.
Can be done after AF takes control, but must be done via AF systems and not DL.