Originally Posted by 1st Class Gekko

Yesterday I tested out this scenario for my ORD - YYZ flight. I booked an AC ticket on UA metal since it was only $200 vs. AC's $600 and arrived at the airport much earlier than my scheduled UA flight. I went to the AC counter to inquire about taking an earlier AC flight. The AC agent directed me to Terminal 1 to talk with UA (I didn't quite understand this since I was on a AC ticket). Anyways at the UA counter I asked the agent to put me on standby for the UA codeshare flight I wanted on AC metal. The agent issued me a boarding pass for the AC flight with no seat assigned and did not charge me.

In the end I saved $400 by booking the UA flight and got to take an earlier flight that was much more expensive for no additional fees.