Happened to me late last year. Traveled dfw-iah-aus for the global entry renewal interview in aus. About a week before, work wanted me to go to ewr the day i was returning from aus. They booked it, aus-ewr-dfw. I did not think much about it, but when I went to check in for the outbound, to aus, could not check in. I was willing to give up the return and not worry about refund or credit. My original was booked thru Chase portal, so UA would not allow me to make a change. The chase rep could not get anybody to answer the phone at UA. So when still on hold with chase, I called the 1k line. They said they could not do anything, chase would have to. So I just had chase cancel it, instead of trying to change or drop the return from that reservation. Ending up canceling the entire dfw-iah-aus round trip and booking a one way so I could make all the flights.

On AA 2 years ago, same type of thing. Work wanted me somewhere, after I had booked a vacation. I was on AA, they booked me also on AA. AA did not make any fuss about it. And what I thought was interesting, (this was when piedmont had all the computer problems), AA gave me extra miles for my inconvenience on a trip I did not take because it was on piedmont.