Chinese visa Category change from L to F Type for Indian Passport.

Hi, i had chinese L type, but we requested F Category " Business F " ( I am not worked at India. So i cant provide any local company sponsor letter. But i had submitted Letter of Invitation Duly Authorized Unit from Chinese company ). So grated only L Category. Is it possible to change " L " category to " F " category at Guangzhou, china. Because i get only single entry and 30 Days maximum can stay. I came to china. If possible, what are the documents i need to submit at Guangzhou Office and where it is? Please advice me.