Originally Posted by
Grouchy
As Baccarat King wrote OP has to get the KLM PNR from Delta. And go to KLM.com and add his DL SM FF number to the booking. KLM will recognize his status and OP will be able to select preferred seats for free. It's way faster and more reliable then calling tweeting KLM/Delta and hope you have an agent who knows what (s)he is doing.
I tried that a month ago for a flight I had recently. Even after getting into the KLM site with the KLM locator the seat changes would not stick.
I finally called the KLM EU phone number and after the CSR confirmed my status, I was all set with the seat of my choice. Interestingly, the day before my flight, the Delta website showed a seat different from what I had selected by phone with KLM. I checked in via the KLM site and I did get the seat I had set up by phone.