After an American becomes a citizen of China, can TA change back to American citizenship?

This mainly depends on whether ta really gave up American citizenship in the process of becoming a citizen of China. In American law, you can't lose your American citizenship if you don't want to give up. So if you want to lose your American citizenship, you must give it up.

Article 8 of the Nationality Law of China stipulates that "a person who has been approved to become a citizen of China shall not retain his foreign nationality." But I'm not sure how to achieve this in China; Did they ask ta to give up the evidence of foreign nationality at any point, or did they just rely on the applicant's own words? If you don't really give up your American citizenship, you will always have American citizenship without doing anything. If you give up your American citizenship, you must apply for immigration, a green card and naturalization, just like all other non-Americans.