How long ago was your trip? Was the DL flight KL-marketed or DL-marketed? This has not been true since they made C+ a separate fare class on DL TATL routes (Jan 22, 2018). Upgrades are now processed into SU class just like domestic. It can't be don't done with AF/KL marketed flights because they have no concept of a SU class for upgrades.
I flew ATL-JFK-AMS-DEL Nov. 16th
DEL-CDG-ATL Dec.9
I booked January 3rd on KLM
ATL-JFK was on DL steal, showed up in the DL app, UG no problem.
JFK-AMS was on DL steal (I think, sorry I've had so many AMS flights this year) and I had to call in to get the C+ seat.
DEL-CDG-ATL were both on AF, I bought preferred seats at booking, when I called in they said I could have gotten them for free with PM.
So, you my be correct, but in may case they did get me into the "Premium Class" after I called.
My original ticketing information was "Economy Class"
It may be different since I booked earlier in January...
