Why is Fang's surname not his father's?

It is said that Jackie Chan's ancestors had something to do with Fang. Later, due to political reasons, he changed his surname to Chen, so he had it. Jackie Chan was born in Hong Kong and went to the first grade when he was a child. In Longge's words, he was hungry except sleeping standing up, so his father sent him to learn martial arts. Sammo Hung, Yuan Biao, Yuan Wah (Bao Gong) and Yuan Qiu (Bao Gong) were all disciples of the Yuan Dynasty, called Yuan Lou, and later he was in Qiduo.