So a very similar thing happened to me two weeks ago only I was flying on a paid J ticket. Arrived at AMS and all the flights to the JFK were delayed. So I tried to move flights. I looked on delta.com and ExpertFlyer and saw that there were J seats available on the flight I wanted. KLM gate agent refused to put me on the flight saying it was sold out. I said, Delta is selling the seat (it was a DL operated AMS-JFK flight). Called Diamond line and they got me on the flight but just like the original poster, they could not issue boarding pass of assign a seat. Said go to the gate agent. Gate agent literally flipped out that I had been put on the flight. I was still on the phone with the Diamond agent and the KLM supervisor wanted my cell phone. I gave her the phone and she literally yelled at the agent for 3 min. I walked away still on the phone. The diamond line put me on a KLM flight and said go to my original gate and ask that agent. When I told the new agent about the yelling and refusal to change my flight despite the booking, thie new agent said she would accept the delta rebooking (I mean it is being sold onlne, accept???). She said the other agent had already put staff on the delta flight which is why she couldn’t accommodate me. So the truth came out. KLM wants to hold seats for staff. Super bad customer experience and really cant believe Delta allows this. They hide behind KLM system vs Delta but the seats are being sold online so it is not that. I haven’t had time to write my complaint, but now I will. Delta needs to look into what the senior ground staff is up to at AMS. Losing the IRROPS ability to change flights while on paid J for staff travel is not cool.