snapshot too old

Hi,

I've been struggling to update this test table that has about 1.5 million rows in it. I'm getting this error message:

01555, 00000, "snapshot too old: rollback segment number %s with name \"%s\" too small"
// *Cause: rollback records needed by a reader for consistent read are
// overwritten by other writers
// *Action: Use larger rollback segments

Below is my script. Any suggestions would be greatly appreciated.

--
declare
     counter number := 0;
     cursor c1 is select ssn from test_table;

begin
   FOR c2 in c1 LOOP
 update test_table
 set SSN = TRANSLATE ('915482376','123456789',TRANSLATE(SSN,
'1234567890','9732581406'))
        where ssn = c2.ssn;

     counter := counter + 1;

     IF (counter = 2500)
     THEN
        COMMIT;
        counter := 0;
     END IF;
   END LOOP;

   COMMIT;
END;


/

exit;