Conversion from F to X visa

Hi,
I'm planning to get F visa from Chinese embassy in my home and when I arrive China I'll find a university to give me x Visa (X1 or X2) and start studying. I couldn't get admission notice to study because of the long holiday. I'm going to study in China from 1/3/2014 to 30/7/2014. My question : is it possible to convert from f visa to X visa?. if not, what types of visas that can be converted to X.
thanks in advance!