Same thing happened to Mom from EWR-DEN (she was flying LGW-EWR-DEN) last Friday. Had a confirmed paid (in J/D) FC ticket and was bumped from FC. Granted her plane was late, but she still did make the connection, and I was somewhat surprised they did not downgrade a gate upgrade (I'm assuming there was one as she did have a FC seat assignment when she left London.)

Moral of the story: CO's FC product = good ^ so put more of it on the planes