jacobus, the only thing that I can find about the xpdr thing is on AIP AD 2.23:
2. Use of Mode S Transponder after Landing
2.1 Aircraft equipped with a ‘weight-on-wheel’ switch must continue to have its transponder operating (on ‘AUTO’ or ‘XPNDR’, and not ‘STDBY’ or ‘OFF’) until fully parked at a stand.
You can also check that on the web:
http://www.hkatc.gov.hk/HK_AIP/AIP/AD/HK_AD2-75to78.pdf
Obviously, it only says that you have to keep your xpdr on, but does not mention anything about not changing your squawk code. But the reality is, if you're squawking the correct code, we can see your callsign on our ground radar, if you change to anything else, your callsign will be replaced by that code (i.e. 2000 in your case). The worst thing is having an a/c turning off its xpdr completely (or squawking sdby), if we're lucky, there's still a blip indicating its position (but nothing else). If there's no blip, and when we get very busy, there's a chance that we could forget the presence of that a/c, which is definitely a safety issue.
2 reasons for asking if an a/c has turned on the xpdr:
1. remind the pilot in a more polite way
2. under some circumstances, the pilot has turned on the xpdr already, but for some reasons, it's not painted on our ground radar
I know it's kind of messy yesterday, but I can assure you that we're all trying our best to get things back to order, and appreciate your (and other's) kind words.