I searched for this, but didn't get a real good answer.

I also called the concierge twice and got two different, and completely conflicting answers. As an aside, this was a rare (first?) instance of where the 1-800 concierge service was completely useless, as well as rude and indifferent. Sigh. I hope it isn't a trend.

Here is the question: traveling SFO-YVR-YYC on AC ticket stock (014). SFO-YVR is UA metal, but I don't think that matters. I want to switch to an earlier direct flight. Only option is UA, as the AC flight is oversold. The UA flight has every fare bucket open--including reward space, but as it is same day they are only showing J class fares to AC. I am on a Flex fare, so it looks to AC like no space is open. So, can I do a SDC or standby to the UA flight?

As I said, AC concierges were making it up as they went. In the end, I just ended up buying a UA fare and cancelling the AC fare. But I am curious as to what the real policy is.