I rang some time ago to have my seat assignment changed to a regular economy seat, having preselected a "Comfort" seat, so that I could sit with a colleague on the same flight.
The KLM agent was very helpful, and changed the seating request as I asked.
2 days later, when I happened to be checking my bookings, I had been reverted back to my originally-selected EC seat.
I guess the lesson here is - I should pay to upgrade my colleague ;-)