I have always sent a message to KLM on Twitter with the confirmation number. I just state I want a EC seat an Aisle. They always get back to me in a few hours to confirm it. I have had some issues with a DL Flight number on a KL flight. KLM does not like that for some reason. When DL assigns the seats on KLM I have had not much luck with the seats sticking. I always just send a message on Twitter and they are always very accommodating and they stick.