I've not had your particular issue, but I've had other issues changing fully changeable Z fare tickets where the flights would not re-price correctly online or would not reprice at all if the change necessitated a ticket exchange.

I was only able to resolve the issues by either

1. Calling AF/KLM in France (perhaps the Netherlands would work as well?) and convincing the agent to to transfer the call to the rate desk (or something like it, I forget what the department is called at AF/KL).

or

2. Stopping by the ticketing desk at the airport.

My experience with the base level agents is that they go by whatever the computer spits out. It was impossible to get them to read the fare rules. Once I got to the right person, they would first check the fare rules and then make the changes without fuss.