According to United Change Flight Policy you can make changes to your tagged flight whenever before the flight's booked takeoff. You don't bring about the expense assuming that you change your trip in no less than 24 hours of booking, or on the other hand assuming that you book your ticket something like 7 days ahead of time and you're flying locally. The expense is higher for global travel, so actually take a look at the Unified site for additional subtleties on that. So as long as you keep those guidelines, you shouldn't need to pay far too much to reschedule your flight. To know more about it read the full Blog.