I noticed something odd last week when I made a change the night before departure. Originally booked in E+, and my MUA cleared 20 days in advance, and got the F seat. The day before departure, I decided to take that evening's redeye and IIRC in the UA app trip options a blue banner type option appeared to change the flight. When I clicked the banner it offered the redeye option as the first (and only?) choice, in F, with zero fare difference. Finding that hard to believe I opened another browser and followed the standard manage flight/change flight flow there with the traditional grid format search results - the grid displayed a ~$271 fare difference. I was a bit puzzled by that, but went back to the "banner" option to change the flight and indeed it processed with zero fees and kept me in F.