As a GM on a DL award ticket on a KLM intra-Europe flight I could assign myself a standard seat in advance, no charge. And then any seat, including the exit row, during OLCI. On the KLM website.

Be advised that sometimes KL will lock the seat map if a flight gets close to full, and then neither you, nor DL, nor a KLM agent can assign anything.

KLM gives you a nice sandwich too.