Hong Kong's status did change, it became a colony of Japan once they conquered it. The Japanese moved all European officials into labor camps outside of the city and repatriated many Chinese immigrants back into China. They also installed Chinese into government positions that were previously held by British officials and the Japanese went to greater lengths to explain policies to the Chinese, but Japanese occupation in the end was much harsher and crueler than British occupation. I don't have the numbers on me, but with harsh Japanese punishments and treatment, lack of resources especially towards the end of the war, and repatriation, Hong Kong's population dropped drastically, I'd say less than half.

The British were actually pretty worried that the Chinese would try to re-establish control since Hong Kong and Britain itself were devastated by the war. However, tensions between the Nationalists under Chiang Kai-shek and the Communists under Mao were already beginning to manifest and though the Americans were allied with Chiang, they were in no position to recover Hong Kong without American help. Chiang began to realize that the British wouldn't give up Hong Kong easily and that failure to do so would discredit him in China.

I'm mostly drawing on John Carroll's A Concise History of Hong Kong and I can follow up with more details if there's anything else you want to know.