Originally Posted by
leftysauce
actually UA will (or at least used to) automatically offer F on paid or upgraded F itineraries but it will also offer options in Y where F isn't available. The difference between before and now is that you can no longer see which flight is in Y or F. You could do a search and look for J count but when you're rebooking, every second is essential.
The website still shows fare classes so...
I think UA app is only available in vertical mode
On DL, I've rebooked onto flights with F space available via the app only to find that I was rebooked in Y. In each case, I was able to get an F seat on the new flight, but it seems like a stupid algorithm instruction to just throw paid F pax into Y when F space is still available.