We refer to it as "early show" and "late show" It can only be done at an airport "ticketing" or "check-in" counter (ie not online/ kiosk or at "baggage drop" - but I think you'll get the self serve ability in the future).

You can "early show" at any time on the day of departure. You can only "late show" (that is, to miss your flight) if you present yourself at the airport or contact the Sales Centre, no more than TWO HOURS AFTER your scheduled departure time.

You can even change the route provided the origin and destination remains the same (so you can go from a connecting flight to a direct route). The fee for either service is a flat $50 (plus $2.50 GST), payable by cash, credit card or debit (WestJet dollars and travel credits cannot be used for fees).

As I said, its a confirmed seat, so standby is not available at this time. I think thats all the questions I've ever had about early show/ late show. If you have any others let me know!