According to the information available on the official website A person can change there preferences and edit their choices filled and after that Choices entered by the candidates shall be treated as final. These choices will be used in the order of preference for seat allotment. The candidates will not be allowed to modify/alter/delete/add more choices after locking their choices or after the last date of choice filling, whichever is earlier.  Therefore, one has to be very careful while filling and finalizing the choices.