Masking Birthday with to_char(to_date(

UPDATE table a
SET a.BIRTHDATE = (
 SELECT to_char(to_date(b.BIRTHDATE,'dd/mm/yyyy'),'dd/mm' || '/1900')
  FROM table b

UPDATE table a
SET a.BIRTHDATE = (
SELECT nvl(to_date(to_char(b.BIRTHDATE
,'MM/DD') || '/1900')
, NULL)
FROM table b

UPDATE tablea a
   SET a.birthdate =
          (SELECT TO_DATE ( TO_CHAR ( b.birthdate, 'dd/mm') || '/1900', 'dd/mm/yyyy')
             FROM tableb b)

begin
  -- Test statements here
  dbms_output.put_line('start: ');
  dbms_output.put_line(TO_DATE( TO_CHAR ( to_date('10-AUG-1951'), 'dd/mm') || '/1921', 'dd/mm/yyyy'));
  dbms_output.put_line('date: ');
end;