Upgrading on KL flights is a shambles: FB can see if they think there there is availability, but as they operate AF systems, they cannot confirm upgrades on KL. Instead, they have to send an email to Amsterdam and KLM then respond (invariably in my experience) by saying 'it is not possible'.
Upgrades on KL flights have effectively disappeared - nobody in their right minds will buy a B or S class ticket to get an upgrade if there is no guarantee of availability.