I've received that message once. However, the original flight remained on my itinerary. So the seemingly odd thing here is that the OP couldn't see the originally scheduled flight.

That said, I wonder if that is the computer's way of not canceling the rest of the OPs itinerary if they don't make the original connection. As we all know, the new UA is very aggressive about canceling the onward segments when a passenger is not onboard the original flight once it is closed.