Actually you can do it in advance, Upgrades can be done up to 6 hours prior to departure. (a hint for you, don't check-in online unless it's 6 hours to departure or when you have already secured the upgrade). If you checked in online while your in economy then you won't be able to upgrade to business in advance.

I would check every now and then to see if there is any availability, If you got the upgrade prior to 24 hours or less to departure, I suggest to call Emirates office in HKG and try to get the chauffeur drive if it's possible. You will also take advantage of the baggage allowance. If your staying in DXB then you can also avail the chauffeur drive.

Upgrades at check-in and onboard does not avail you the chauffeur drive in DXB.

Though the loads between HKG and DXB is always full in my experiance.