1 upgrade cert is good for origin to destination including connections. Check your ticket to see if it shows YVR, YYJ, YVR as a connection on your way to SYD; if not, then you'll need 2 cert. One for JFK-YVR and another for YVR-SYD. Assuming it shows as a cnx, then if your JFK-YVR cleared at ckin, then they can notate your record so agent won't ask for another one in YVR.