Working to studying to working - how to handle visas?

[xiaorui]

Have read various conflicting things about changing visas, so thought I'd see if anyone can offer advice on my specific situation. I'm a UK national currently working in Beijing, teaching English at a private business school on a Z visa. My contract ends July 2011, which is also when my visa expires. However, I'm thinking of quitting my job in Jan/Feb in order to study Chinese for a semester at a university, possibly Beishida or BUAA. After that I want to look for another job in Beijing, and don't really want to have to leave mainland China for visa purposes at any time in the next year if possible.

So, first question is, do employers automatically revoke Z visas/residence permits when an employee quits? Any kind of standard procedure for this? I'm assuming they will, but wondered if anyone has experience of this.

Next question is: what is the procedure for changing from a Z visa to an X or F visa? I've emailed a few universities to ask about this but haven't heard back from them. I've read different things about needing or not needing to leave the country to do this, and also about whether a student only studying for one semester should have an X visa or an F visa. Can anyone shed any light on this?

Then what I'm a bit more worried about is changing back from an X or F visa to a Z when I (hopefully) find a job after studying. From what I've read, it is necessary to leave the country to do this.

So my other option (I think) would be to pay a visa agency to extend my Z visa so that I don't then have to go to all the trouble involved in getting a new Z visa when I finish studying. In which case, is it worth the money, and how do I find a reliable visa agency, and is it ok to study on a Z visa?

Advice on any aspect of this will be much appreciated!