As per the thread subject, can I do this?

I have a flight YYZ YOW on Tuesday. I really have no need now to fly at my scheduled time. There's plenty of seats on later flights. Does anyone know...

a) Why can't I stand by for a later flight? I know you've never been able to do this, but why not? I can standby for an earlier flight, so if I opted to fly later, someone who wants to fly earlier could take my seat

b) What happens if I miss my flight? Can I just go to the airport and get a new BP for a later flight?

I'm trying to minimize my spend with AC this year, so paying a change fee is something I now no longer want to do