So, I've been in an interesting situation today, and would like to get up-to-speed on the code-share rules. I fly 99% UA metal - however, for my trip to YYZ this week, UA flights were almost twice the AC flights, so I booked AC. AC was on AC metal ORD-YYZ and UX metal YYZ-ORD - but AC-issued ticket.

Showed up at YYZ today fairly early and asked at the Maple Leaf lounge if I could switch to the earlier AC flight. They said I could for $75. I passed, then in 2 minutes I got a txt saying my flight is now delayed until 9pm, and it was 4:45pm at the time. So I decided that I don't want to spend that much time in YYZ. I went back to the AC agent. They called UA to offload me of the original UA flight I was on, put me on AC flight and charged me $75 change fee. Well, I landed at ORD(later than expected on the original flight, but at least it was reasonable), and got a txt that my UX YYZ-ORD leg is cancelled and I was automatically rebooked on the UX flight next morning.

Now, despite the fact that I did pro-active change, if I stuck it out in YYZ, would UA be responsible for my accommodations or AC? Is there any recourse against AC for charging me $75 fee? If I book a code-share flight, does the ticket-issuing airline have any responsibility?

Buyer's remorse? You agreed to pay the $75 and I think you should move on then.

No, it's not buyer's remorse. Rather, I am trying to understand what the rules are in the codeshare situation and who has responsibility. Since I usually fly UA metal exclusively, I've never been in this situation during IRROPs. Plus, since my flight ultimately got cancelled, I am wondering what the rules are in this case.