What is the ruling on changing one's madhhab from Sunni to Shi'i (the Zaydi denomination), and what is the difference between them?

It is obligatory for a Muslim to follow the Qur'an and the Sunnah (definition) according to the understanding of the righteous predecessors. The Prophet, peace and blessings be upon him, informed that the Ummah would divide into seventy-three sects, all of which would be in the Hellfire except one, which is the Jama'ah (the community), "that which I and my companions are upon." Therefore, following the Sunnah is not optional. The Zaydis are a sect of the Shi'a, attributed to Zayd ibn Ali Zayn al-Abidin, and they differ from the Twelver Shi'a in the order of the Imamate. Among the beliefs of the Zaydis are: the belief in the awaited Mahdi, the eternal damnation of the perpetrator of major sins in Hellfire, the preference of Ali over the two Shaykhs (Abu Bakr and Umar), some of them believe in the Prophet's designation of the Imams and their infallibility, and their rejection of the caliphates of Abu Bakr and Umar and disavowal of them. Ahl al-Sunnah (Sunnis) differ with them in these beliefs, as they do not believe in the Prophet's designation of Imams from his household, nor in the infallibility of anyone except the Prophets. The perpetrator of major sins is under Allah's will, and the best of the Ummah after the Messenger of Allah are Abu Bakr, then Umar, then Uthman, then Ali.