Please advise converting Z to X... work contract will be finished, want to study!

It seems most people convert x (study) to a Z (work) but does anyone know about converting a Z to and X? Lets say you are working on your Z and after your contract term is up, you want to do some intensive Chinese studies ... can you do this without leaving China, say before your Z expires? I would like to do this but don't know the answer. Thanks in advance!