You have to go to the Local Exit & Enter Department of Public Security Bureau to apply for extension of L visa before the expiry date. Once L visa expires, you will overstay in China and will be fined for 500 RMB for each day. It costs RMB 160 for vsia extension. It is possible for change L visa to F visa, but you have to prepare all requirements for F visa application, such as Invitation Letter issued by Foreign Affairs Office