Actually, the conversion is " to apply for a new Z visa".Documents required:
1) One completed Visa Application Form with a passport photograph attached;

(2) Your passport with at least 6-month validity and at least 2 blank visa pages;

(3) Employment Permit (the original one and a photocopy are needed), and "Visa Notification" letter (the original one and a photocopy are needed), which are both from the competent office of the Chinese government.

(4) Physical Examination Record For Foreigners completed by local public hospital.

If you are going to teach in China, working license issued by the Bureau of Foreign Experts of relevant province or municipality (the original one and a photocopy) is needed.