Because you are using two seperate tickets, you are allowed to do as long as you do not use the first sector of your M class ticket. I take it you're using B class YVR-HKG, and then onto Australia on the M class ticket. (How may I ask is the routing from HKG-YVR to go onward to Australia? HKG-YVR-HKG-SYD-HKG?)

If you decide to do it, you must cancel the HKG-YVR in M class as you have no intention to use it anyway. Just don't ever use it in reverse after your Australian trip ! You are allowed to 'mix' reservation classes in one booking PNR. (I do it all the time).