what is the rule to convert X visa to Z type?

If you have found a company to employ you, then your employer will get you the alien employment permit and invitation letter from a duly authorized unit.
Next, prepare all the required materials for work Z type containing your passport, valid registration form of temporary residence, the application form attached with a half-length recently-taken color photo, as well as the alien employment permit and invitation letter from a duly authorized unit.
Finally, submit the application to the exit and entry administration of the local public security bureau to convert X type to work Z type.