Short answer: yes, you can upgrade your booking class. I have done so in the past. It will cost you the fare difference plus the $200 change fee associated with your L fare.

Longer answer: note that H fares are expensive inside 7 days of your departing flight. The base fare for H from YUL-SYD leaving in two days is 2688, or about 600-900 more than your L fare. The fare rules allow you to combine the economy classes on half round trip bases. In other words, you could reprice your ticket so that the outbound is in H and the return is in whatever is cheapest right now (currently V9 Q1 L0). So you could shave about $200 off the cost of going to H in both directions.

And put yourself on the upgrade waitlist as soon as you re-ticket.

So, $800-$1000 all told to move up to H class on two days' notice.