Withdrawing vakaltnama

You can do so. You can take NOC from earlier lawyer if he agrees then there is no problem. If he don't then you can file an application for recusing earlier lawyer and entering appearance of new lawyer. Your new lawyer may be aware of this procedure. Nowadays, Court instead of taking NOC, accepting new vakalatnama only upon mentioning. 

Usually the advocate who was engaged for the case refuse to give NOC unless the complete payment of the case given to him and even move application in the court regarding this, so better clear all dues with you lawyer first before engaging new lawyer for your case.

You can appoint any lawyer of your choice who shall in turn file his/her new vakalatnama in the case. It is advised to take NOC from previous lawyer also.