There are still isolated valleys and uplands with seasonal butterflies, also thick forest has some excellent species almost year-round. A butterfly trip to HK should include Taiwan or China though, if that is the main purpose of the trip.
Butterfly Valley in Kowloon is now the ‘proud’ owner of a covered reservoir, road, fly-over and government works buildings. So the renown valley, salivated over in turn of the century folios exists now in name only.