I regularly fly into ORD around 7pm and connect to YYZ at 9pm on thursdays.

At 7:45pm UA codeshared / Air Canada operated flight 512 departs for YYZ, and I'd prefer to get home early. My procedure for switching to this flight has been:

1) Ask AC GA if theres space, they also check my ticket to ensure that it's endorsable.
2) I head over to united CS and have them offload from me the 9pm united flight.
3) I return to the AC GA and they issue me a boarding pass.

All has gone well the 5-7 times I've done it. Until yesterday.

I arrived at the F2 CS Desk and handed the agent my boarding pass, stating that I'd like to be offloaded. She looked at my boarding pass (S class) and said that because it is non-refundable and non-changeable, she could not offload me. This is without actually looking up the fare rules in any way. I said that I was over at the AC desk and they said it was an endorsable fare.

I was then told "This is a new policy by United". Then, I explained that I have done this many times before and as early as last week and I doubt the policy has changed that quickly.

Next, I got the answer of "If I offload you, It would cost United revenue; Air Canada gets the fare and united gets nothing. This is a ticket issued by united and on united metal. You are just going to have to wait for your united flight." There was another CS agent beside her nodding and agreeing with what I was being told.

I realized that I was not going to get anywhere with these agents and I remembered the flyertalk golden rule "If you aren't getting the answer you'd like from an CS agent, find another one".

I thanked them for their time and took my ticket to the RCC next door and the agent offloaded me without question.

I admit that I don't understand how airlines get compensated for these kinds of ticket switches, and I can understand the (poor) logic of the agent trying to play a hard game of defense to retain revenue. What I really dislike is that United sells me fares with this provision so bbviously, people at revenue management are fine with the ability of united pax to leverage the star alliance. Who is the CS agent to then tell United that they're wrong to offer this service and to try and pass it off as an official policy change?

I just wish that these kinds of vigilantes could be weeded out. Why do I, as a customer of united (heck, any airline), need to know MORE about the rules, policies and procedures than the employees serving me? In order to protect my own rights, I have to and it gets annoying over time.