Can I get a F if I am already in China under an L?

I am currently living in Shanghai with an L visiting my family. I was offered an internship position as a junior programmer at a small foreign company based here. Can I switch to an F? The worry is that the police come and do their inspections at the office and tell me I can't be there are kick me out of the country. However, if I am not taking a salary then it shouldn't be a problem right?