As I see it, there are two possibilities here as two what happened: Either the OP misread his itinerary and/or the flight schedule, in which case there might have been an aircraft change indeed and he moved from an entitled F in an F/Y configuration to an entitled C in an F/C/Y configuration. Or, other option, the check-in agent at his point of origin wanted to do him a favor by putting him in three-class F (or made an unlikely mistake) and the gate agent in ORD back him out of it. In the latter case, though, I am wondering what the talk about an equipment change is all about. They should at least be honest about it. My personal guess: Option 2 and feeding the customer a line so as to not upset him. Which, in that case, did not work.