I tried this in ORD a couple of years ago - I was ticketed on UA for a flight to YYZ and there was a AC flight an hour earlier. Since I was at the airport early, I approached UA customer service counter about catching the AC flight, and it was a no-go...the agent was nice enough but something about UA would have to fork my fare over to AC. Not sure if that's accurate but the sense was since there was nothing wrong with the UA flight I was booked on an hour later, the nice agent wasn't biting.