Yes, thats really true. The chinese government had stopped issuing L and F visas, except for a case to case basis...that is according to the information from the Chinese government. Regarding your problem, it depends...if you are teaching in a school, they should be the one to change ur F visa to Z or residence. But if your school doesn't have permit then find a school that is legal to change your visa to Z or residence otherwise the only thing for you to do is to go back to your country, then apply there,its not even easy because they still need pertinent documents proving the kind of visa you are applying for.