Definitely talk to UA.
I had it where a flight was canceled, an the default flight they put me on basically lost me an entire day with my client. This would not work for me at all.
I just explained to them that I needed the next available flight. No fighting, they (and I) found an American flight leaving in an hour and a half, and they got me on that flight without issue.
The automatic system I believe only puts you on UA flights, and the next UA flight might not work out, so call them.