UA257 Dec. 18/19, F0? P.S. JFK-SFO

Me +2 are currently booked on the 19th, JFK-SFO.

Wanted to change to the same flight on the 18th, but F0 means I can't make the change. So I checked my flight and it's also F0. Therefore I was wondering whether it's an uber-popular flight or the aircraft was switched to one of the new ones without F?

Any insights?

Thanks!