Hi John,
Bill is absolutely right, they are almost the same instrument with just a few sounds between them. The expansion board Bill mentioned is the EW65NX and basically gives the KN6000 the same sounds as the KN6500. The EW01 is the pop production board, which can add sounds to the KN6000, KN6500 or KN7000.
I would also add that these keyboards are so close to the KN7000 that it would make no difference to most of us. I know we love our KN7000s but at the price the KN6000 and KN6500 go for they are a steal and would be a perfect backup for those who are nervous about the age of their KN7000s!
They also have a great "sound editing" ball that, in my opinion, gives them better (easier) sound editing capabilities than the KN7000.
The best compatibility information between the Technics instruments that I can find is on the following page and I'll be keeping it up to date with any new findings:
http://technicskn7000.com/technics-instruments/Further compatibility info is provided by Alec Pagida on Gunnar Jonny's site:
http://gj.technicskeyboards.com/technical/alecs-kn-corner/Best wishes,
Mike