Originally Posted by xliioper

Is this a DL flight or a KL flight (DL cabin is Comfort, while KL equivalent is Economy Comfort)? It seems you crossposted this in KL forum -- What to Do if Economy Comfort Seats Don't Become Free for Delta Gold at 72 Hour Mark?

If you are on a KL marketed, DL operated flight, you are not eligible for DL Comfort upgrades at normal Medallion upgrade windows. This is because the Comfort cabin is separate class on DL (W/S booking classes) and not just extra legroom seats in coach. DL comp Medallion Comfort upgrades process into the DL SU upgrade class (comp Medallion FC upgrades use OU upgrade class). But the system will not process these on KL coded flights as you can't have a DL class on a KL coded flight. Comp medallion upgrades on KL marketed, DL operated flights are only offered on day of departure.

If this is a KL operated flight, then DL does not list upgrade windows for Medallions for KL Economy Comfort seats. They only say it is complimentary for DMs and Plats and available at a reduced fee for Golds and Silvers.
"(4) KLM Economy Comfort is complimentary for Diamond and Platinum Medallion Members, and is available for Gold and Silver Medallion Members for a reduced fee. "
https://www.delta.com/us/en/skymiles...yteam-benefits