Just completed my check in for my award flight from AMS-JFK via CDG on Air France. Ticket was booked in class X, and I got the chance to upgrade the CDG-JFK section for 286 euro on the AF site.
I took it. Choose 66L. When processing the payment an error shows on the screen. When checking de KLM site to print the boarding passes, I expected my old Y seat. But to my surprise I was put in 72L.^
So, to answer the question. It is possible to upgrade on an award ticket
