Changing from single to multiple entry visa

I have a X1 single entry (I will be studying for about 4 months), and I was told that upon arriving in China this can be switched to a multiple entry one.

I am wondering about how long this process takes, or if for a fee it can be expedited, since I would like to join my family to Japan 2 weeks after I enter PRC.

I arrive in Shanghai the 8th of September and plan to fly out on the 21st... returning on the 5th of October. Will this be possible? The visa office in Sydney said it would be, But were not much help. but I am nervous about the short time.... Thank you!