Actually the 2 highest KLM economy fare classes are S and B (eg Y = S).

You need to have "D" inventory available for the upgrade. This is much more limited than the upgrade buckets for KL/NW miles. It seems that "D" gets released only a few days before the flight (or maybe as much as a week beforehand !). I found that the easiest way to check for D availability was to use:
www.flyaow.com/classamex.htm
(If D is not available, it won't show up at all).