How to change alien employment permit to expert certificate

Hi, I was once a foreign teacher in hangzhou but I finished my contract and cancelled my expert certificate and moved to Shenzhen for a company work. The new company got me an alien employment permit and new residence permit. But now I want to go back to my previous school for my teaching job.
How can I change my alien employment permit to the new employer so that they can apply new foreign expert certificate for me? My current residence permit is valid to November 15th . I want to change the employer without leaving china. Thanks