In my experience, if I have to change currency (and cannot withdraw cash as the other answers suggest), usually the rates seemed better at the country of departure, probably because you can choose to do it outside of airports, so more choice means better rates.
However, the one exception I've encountered was Singapore: exchanging Euros there seemed much, much better than buying Singapore dollars in Europe. But Singapore is a hub country full of traveling foreigners, which probably explains it.
If Hong Kong follows a similar pattern, then it could be better to sell euros there, instead of buying HKD in Europe.