All MileagePlus Premier members can confirm a seat on a number of other flights at no additional cost, according to the new united same day flight change policy. The only prerequisite is that your flight takes occur within 24 hours of the originally scheduled travel and has the same origin and destination. While basic economy ticket holders cannot change their flights, they may be put on standby for other flights that depart within 24 hours of their original travel time.
https://airlinespolicy.com/fli....ght-change-policy/un