switching employer in china. what to do?

Hello,
i have a question regarding switching employer in China.
Originally i arrived to Mainland (Shanghai) with a Z visa which was then converted into a regular working permit . i am not about to change position , company and city.

I understood that i should now cancel my working permit in Shanghai. but what happens when i apply the new working permit?what documents should i and the new employer prepare? can i start working meanwhile my application is in processing or i must wait to get the the new working permit (it generally takes 20-30 working days)?
thanks