I booked my son a 3 leg trip on United and he later told me he already had a train booked for the third leg/trip home. Can he just call United and ask them whether it's best to change the trip to eliminate the 3rd leg, or should he just not show up for the final leg? I seem to recall (maybe it's poor memory, or just a Delta thing) that sometimes changing a booking means the whole trip is rebooked and it can be more than just dropping a leg. Or maybe I'm completely wrong.

Any help would be appreciated!