Not a published practice but reportedly GG FLAT TIRE (
as of April 2018)
If all UA flights, UA will generally try to make something work. If you show up late, don't lie, don't make up a far fetch story, airport agents can tell when they are being played. Just say, I messed up and what happens now? Ask for help, don't demand it.
What will generally happen is you will be placed on standby (as a revenue passenger), and may have to standby for a few flights, but short of a major irrops situation, you will be eventually accommodated. In some middle E- seat.
Don't depend on seatmaps to think there is space. In the days of Basic Economy, advance seatmaps in economy are works of fiction.
This is not a good habit to form but as a one-off, you will get accommodated eventually ( maybe the next day)
SDCing the day before to a later find, if possible, is a much better option.