The procedure for a gender reassignment is a long one and the certificate for the change of sex is given only after the completion of the procedures. The certificates from two independent Psychiatrists, the Plastic surgeon who performed the surgery have to be attached when appearing before the Magistrate to apply for a gender change and then you can give out the notification in the gazette.
That is the proper method to go about the procedure if you want the legal formalities to be completed. You cannot legally opt to change your gender before undergoing the transformation.