Officially, it is listed as "day of departure" eligibility. But the odds of any decent C+ seats still be open then will be slim. The reason advance upgrades won't be processed is that DL C+ is a separate fare class (and not just extra legroom seats) just like FC cabins and they must process into DL specific comp upgrade classes (SU for C+, and OU for FC comp upgrades). These upgrade classes are specific to DL coded flights and you can't have AF/KL coded flights with SU/OU classes on them. With gate/day of departure upgrades, they can potentially put you in these seats without changing the booked class on flight.



While AF/KL "Light Fares" do not have a separate fare class like DL's BE fares, the airlines can identify them by their fare basis codes which have a 'B' in the second to last letter just like DL BE fares. DL Main Cabin and AF/KL Economy fares will have an 'M' in the second to last letter of fare basis codes. You can readily see the coordination of fare filings as part of JV in ExpertFlyer fare listings. Here are LAX-AMS fares on DL/KL below. Note that access to Sky Clubs with a SC membership or credit cards is explicitly prohibited on both DL BE fares and "similar" partner light fares. And, as noted above, SC access on international Skyteam flights with DL STE+ status isn't allowed unless you are on DL FC/PS fares or partner PE fares (simply booking international flights on partner Light/Economy fares won't get you SC access as a DL STE+).