I assume you have no AY+ account (so no points to buy an upgrade for)?

In that case only 2 options, both not very likely for a HKG flight:
1. AY offers to upgrade you for money and you accept and pay. Follow this thread to understand how it works:
http://www.flyertalk.com/forum/finna...de-offers.html

2. The flights are overbooked and AY offers to upgrade you for free to avoid denying someone boarding.
The chances of this are very very slim.
Op-ups are most of the time handled by staff at the airport, so normal rules apply to maximize chances - check in early, be at airport in time, travel alone, et.c.