Originally Posted by HalconBCN

If I were you, I would bring the printed Delta confirmation with you to the airport. KLM will be obliged to honor the service you have paid for, whether that means giving you your originally selected seat, an alternative window seat (I'm sure there will be one available, as you are only one person)... or maybe an even better seat in the so-called Comfort section.
And if KLM didn't honor the selected seat, you can claim the cash back from Delta. Win win situation!