I've had this scenario happen to me a couple of times earlier this year (first MIA-GRU and later GRU-MIA)—the first time I did exactly what you suggested but they had new boarding passes waiting for me at the gate. I tried to argue and all but got nowhere, so I just took the new boarding passes and thankfully was able to switch seats with people onboard (they had given me and my wife seats on opposite sides.)

When it happened again on GRU-MIA I didn't want to risk it so I just called in and got re-routed thru DFW.

Of course, YMMV, but bottom line is: I wouldn't count on being seated on the F cabin in these cases.