to change employer, release and rec letters required? old permit not canceled vs already canceled

To change employers (schools, within the same city), I understand I need a release letter indicating that I no longer work for the old employer. Do I also need a recommendation letter (wai4guo2 wen2jiao4 zhuan1jia1...tui1jian4xin4)? If so, must it be from my most recent employer? Or can it be from a previous Chinese employer? Old employer has not yet canceled my current work permit, but they are on the verge of doing it.

Is this process any different when the current work permit hasn't been canceled yet, compared with when it has already been canceled? Thanks for any help.