changing employers with Z visa

I would like to know what is the procedure to change employers on Z type.

Getting into the specifics I am curious what happens if I get into an argument with present company and he cancels my residence permit, or tries to block my movement to a new employer? How much time do I have while my permit is still valid, can I go to Hong Kong and apply for an L type in a hurry? If I am able do this, is there any chance that I can stay for 3 months, and after I found a new employer (which willing to give me a Z type again) can I use my old Z type documents to make the new procedure easier? I mean I hope I don't need to go back to my home country again, because when I changed from L to Z, I was told that I must go back and apply at the home country embassy, so I did so, but I hope I can save a round trip this time.

Besides of this I am interested what happens if I change professions, my current Z is for a sales and trading company, but I am thinking of going for an English teacher job. Is it a problem that the job title is different, because I heard that the Chinese government made some more strict laws and regulations regarding to the teacher's application procedure. Honestly feeling a bit worried if I will find a new better employer in the future and agree with them behind the scenes, then want to leave my job and my present employer won't be happy because he paid the Z costs and will try to block the move and make my life even harder. Thanks in advance.