I was under the impression that once you booked on UA metal (even on ac.com), you cannot change to an earlier flight on AC metal. Is this true?

Specifics: I want to fly from ORD to YYZ. The AC metal flights are usually 2-3X the price of the UA metal. Can I book a UA flight, and then stand by for an earlier AC flight? When I flew back from ORD last week, my seatmate told me he regularly changes from UA metal to AC metal. It seems to me that when I've tried to do that, UA will gladly put me on an earlier flight, but always on their own metal. (I will go out of my way to fly AC metal for the bonus miles.)

I also forgot to ask my seatmate if he had to pay a change fee. If this can be done, is there a change fee if booked on ac.com? What about if booked on ua.com, as they usually offer stand by at no cost?

TIA