Still wonder if Delta knows what a mess it really was. Quit often the miles or GUC was never pulled, especially when pax did go to the gate and the upgrade process took place there.
Or after the introduction of C+ and W fares, KL agents selling upgrades to Delta One to their FB members for 10.000 miles;
El cheapo KLM 074 ticket with a V fare or so + EC seat on a Delta operated flight = W fare = Premium Economy, Fully Flexible ticket (AF-KLM)