I have a valid US visa on my Indian passport. I changed my nationality to HKSAR. Can my US visa be transferred to my HKSAR passport?
No. A US visa can not be transferred to any other passport. But if an expired passport contains it and there is a new passport the same visa can be used after meeting certain conditions listed below.
My old passport has already expired. My visa to travel to the United States is still valid but in my expired passport. Do I need to apply for a new visa with my new passport?
No. If your visa is still valid you can travel to the United States with your two passports, as long as the visa is valid, not damaged, and is the appropriate type of visa required for your principal purpose of travel. (Example: tourist visa, when your principal purpose of travel is tourism). Both passports (the valid and the expired one with the visa) should be from the same country and type (Example: both Uruguayan regular passports, both official passports, etc.). When you arrive at the U.S. port-of-entry (POE, generally an airport or land border) the Customs and Border Protection Immigration Officer will check your visa in the old passport and if s/he decides to admit you into the United States they will stamp your new passport with an admission stamp along with the annotation "VIOPP" (visa in other passport). Do not try to remove the visa from your old passport and stick it into the new valid passport. If you do so, your visa will no longer be valid.
(The emphasis is mine)
Source: US Department of State
Your visa on an expired Indian passport is not valid on an HKSAR passport (or any other country for that matter).
TL;DR: you will need to get a new visa
As the other answer points out, you could use both passports if they had been issued by the same country and of the same type. But since you changed nationalities, this is not an option for you and you will need a new visa.
Do take note of the passport number of your Indian passport, in case you're asked to surrender it to Indian authorities. It might help your visa application using your HK passport, as I believe you're asked about previously held nationalities and passports in the application form