L to z visa ! what is the procedure ?

I have been in China since 2004 and i always agents to extend my visa for me after i cancelled my contract with a college i was teaching. now i have a foreign service company and wants to employ me as the assistant manager. Now i have L so what is the procedure to get working and residence permit since i am in china now? Can L be changed to Z without neccessary getting F first ? Please i need your advise. thanks