Up until last week this appeared on the BA FAQs:
I’m booked to/from Hong Kong, can I rebook or refund my flight?
If you are flying to/from Hong Kong up
until 31 May 2020, you can rebook on to another flight, at a later date. It needs to be to the same destination and operated by British Airways.
To make this change please
contact us. If you made this booking via a travel agent, please contact them to rebook your flight.
It appears to have disappeared. Are they no longer allowing for free changes to HKG flights which were booked before 3 March?