Remember, HighFlyah, gate agents hold a lot of power in the UA system (even if some of it has been stripped away lately).

Six months ago on a LHR-ORD-DEN flight when I arrived in ORD I marched up to the agent working the ORD-COS flight. I handed her my ORD-DEN boarding pass with a smile and said, "Hi! I'm on a later ORD-DEN flight, but the COS flight will put me a lot closer to home, a lot sooner. Can I get on the flight if there is room?" She smiled back, took my boarding pass, flicked a few keys and handed me an ORD-COS boarding pass. COS/DEN are not even considered co-terminals, and flying the same itinerary out of COS would have cost me much more. But, there were seats on the plane. YMMV.