This is quite interesting. I put the flight info in Experflyer to see the seatmap.. It is for sure switched to the 777XI/XC (United's Worldwide 2) just by looking at the seatmap . Y looks very open. C cabin has not one assigned seat in it. When i type in F, A or P it will not show a seatmap.. Odd... Anyways, if you paid for a F ticket you should be seating in the FC suites, if you used upgrade certs than expect to be seated in Biz class. This has happened to me before. I was booked on a LAX-ORD on a 757 and that same hour or so during boarding the plane was switched to a 747..well there were pax that were upgraded that had F boarding passes..the GAs were calling a bunch of names tearing up BPs and bounching us back into Biz...no double upgrade