Changing visa types, possible?

I am on a 1year student type in Chengdu with my son. My wife is on a family type to visit her parents in Nanjing expiring in 2017. Mine is ending in July and I would like to change over to join her on her family visa with our son. What documents would I have to bring to the Nanjing entry and exit bureau to have that done? Btw we are all britsh citizens but her parents are chinese.