I regularly standby for different flights on my way home, simply to insure upgrades. Usually, I just show up for an earlier flight, and ask if there are any upgrade seats available, "right from the beginning." (If there are upgrade seats, that infers that I can get on the plane, upgrade or not. But I want the upgrade, otherwise, no thanks.) Flying out of Halifax, Montreal, Ottawa, and recently Quebec City, this has worked well.
However, it has never worked for me leaving Vancouver, heading east. (I always get my confirmed advance upgrade, one way or another, but I prefer certain planes, flights, etc. Sometimes I go to the airport early just for this reason.) But each time, I have been told that previously ticketed/reserved passengers, who request standby upgrades, will be upgraded first. This usually results in the agent saying something like "Well, there are 5 seats open, and 19 on the standby upgrade list. You'll be after them." So it's back to the DMX Music Station for me.
As far as your original reservation goes, it will remain intact until you actually get on a plane. I did this on the way home from Toronto a few weeks ago. I was confirmed upgraded on the 5:45pm flight, but I wanted to get home early. They put me on standby on several different flights, starting at 12:55pm, and even printed boarding cards. I just went to the gate at the right time, and asked if there were any upgrade seats available. Each time they said no, and I just tore up another boarding pass. My originally reserved seat remained intact, and that was the one I finally flew home in.
In case it wasn't clear above, my success rate has been 100% westbound, 0% eastbound.
And bad news regarding AC -- they have already begun allowing people to refund normally NONREF tickets, in anticipation of the strike. I suspect this will mean that AC pax are already switching to CP, also in anticipation. But more importantly, it suggests AC is expecting a strike. Rats!
Regards,
Ken Hamer