I called WS to find out what the standby/change policies are:
Same Day (at least for earlier flight): No change fee if on a standby basis. If a confirmed change is required, $20 change fee + difference in fare (not sure if there is a refund if there is a negative difference).
Different Day: No standby. Confirmed change in reservation required. $20 change fee + difference in fare.
Drifting aimlessly between YVR and YLW