My guess is the change is to prevent KL from filling J seats with paid upgrades and non revs too quickly (beacause KL has control). The last time I was in AMS, my DL flight was delayed. I asked at another gate to be put on another DL flight to JFK. The agent refused even though 2 J tickets were still for sale. So I just called the DL US number and got on the flight (I was on a paid J ticket). When I went back to the gate, the KL agent went crazy (asked for my phone and yelled at the DL phone agent). So my guess is DL wants to retain more control over last min J availability even though KL has control. I still don’t understand how DL’s phone line and websites show seats available (and sell them) but KL gate agents see them taken. I think this is what is driving the change.