All airlines offer the ability to change an economy ticket...dates/times, etc. You need to understand that economy tickets come in many flavours, just like cars can be bought with many options. A car costing $10,000 in its basic form might cost $16,000 once you've added options like leather seats, metallic paint, and navigation systems. It's the same with airline tickets.

A basic HKG to NYC ticket might cost US$900 if bought in advance, but one with greater flexibility might cost $1,500 or more. Depending on what options you really need, and how much you REALLY need the flexibility, it is often better to buy the cheapest fare that allows some flexibility and pay for the surcharges if you really need them.

I can have a quick search for you if you give me some dates.