https://community.servicetitan.com/t5/General-Office/UA-Help-Line-888-82914-67-Why-won-t-United-let-me-change-my/m-p/57018#M4059
https://community.servicetitan.com/t5/General-Office/888-quot-829-quot-14-67-How-long-do-I-have
Additionally, changes can be restricted based on how close you are to your flight's departure time. If your flight is imminent—typically within a few hours—airlines often limit the ability to modify your reservation. Operational issues or a lack of available seats on alternative flights can also hinder your ability to change plans.