Were you allowed to change your religion in Ancient China?

No, they were not. Once they chose to believe in a religion, they should insist in that. If they would like to change, they would be punished, by kicking out of the religion school or even being killed.

thank you so much

The previous answer is absurd， I am going to tell you the truth as I am Chinese， first，there is no term called "changing belief" in Chinese culture, because we are believing in three religions at the same time, from our birth to death. Our culture are really tolerant, so most Buddhists except real monks are allowed go into Taoism temple and vice versa. And Confucianism is the common belief for all Han Chinese.