From the terms and conditions:


3. Refunds

3.1 In the event of an extra comfortable seat not being provided to you, you are entitled to a refund, representing the amount paid for the extra comfortable seats, for that specific part of the journey in the following cases:

• You request a refund of your ticket and extra comfortable seat within 24 hours of booking both.
• Your KLM flight was cancelled.
• The type of aircraft or seat layout for your KLM flight changed, and the same extra comfortable seat type was no longer available.
• You missed your KLM intercontinental flight following a previous flight operated by KLM or another SkyTeam partner.
• You did not use the extra comfortable seat because you changed your flight, but the same type of extra comfortable seat was unavailable on the new flight.
• We used our right to assign or reassign your extra comfortable seat before or after boarding for operational, safety or security reasons. The final decision resides with the check-in agents and the operating crew on the day of the flight.
• It seems this was the case, right? You should have an invoice for the extra options in your email.
  
  I would advise to send this invoice /reservation, together with a copy of the boarding passes where the actual seats where assigned and referring to the conditions stated above.