Back in the days KLM FD agents were able to overbook award inventory for Platinum and Gold Elites.If the D bucket showed zero, they could take seats out of the regular revenue "C" class bucket.
However this is not the case anymore, ironically enough I tried to book two seats in C on a AMS-MIA flight yesterday.Flight showed Z4 C6 J6 and I was not able to get a WBC seat !!