Is it possible to change a visiting visa to a Work Z while in China?

A school needs my services immediately and cannot afford to wait for the 6 week or so process of obtaining a work Z. They have suggested to come over on a visiting type and assured me they would be able to transfer this to a work Z. Some sites say that it is impossible to do this has you must apply for a work Z in your own home country. If it makes a difference the job is very close to Hong Kong. Also, if this is possible, should I have a letter of intent to "volunteer" from the school or something else? My concern is that I will not have a return flight or proof of hotel stay to in fact make it look like I'm just visiting. Thanks for your help in advance.