I've written about this before, but be sure to check your United Airlines reservations weekly, as well as other airlines, too.
We've had multiple changes, loss of reservations from their system, and actual change of airline last week on an interisland flight in Hawaii; Island Air, through whom United had booked us, actually called us to tell us that our interisland flight would now be on Hawaiian Air 12/17. Yeah for Island Air.
We have never received one e-mail or call from United when a change occurs, so the only way to be aware of a change is to sign into your reservation and check it. They seem to put a RED notification on the itinerary when it's called up to notify that way there has been a change, and you have to check at the bottom that you accept the change--if you do! If not, don't check and call United.