I am just curious to know if UA airport agents are able to rebook someone on an AC codeshare flight. I understand they need to be "at fault", they have policies and blah blah blah...

From my personal experience, UA phone agents are unable to rebook someone on an AC flight even if there is some major schedule change or flight cancellation. (Phone agents are clearly unable to rebook someone on a AC codeshare flight, not solely "unwilling".)

By "rebook", I mean "rebook for free".