-- ALTER TABLE ADD TMP_FIELDTOFIX SAMEDATATYPE NEWDEFAULTVALUE [NOT NULL, etc.];

UPDATE RDB$RELATION_FIELDS
 SET RDB$DEFAULT_VALUE =
  (
     SELECT Q1.RDB$DEFAULT_VALUE
     FROM RDB$RELATION_FIELDS Q1
     WHERE (Q1.RDB$RELATION_NAME = 'TABLENAME') AND
           (Q1.RDB$FIELD_NAME = 'TMP_FIELDTOFIX')
   ),
   RDB$DEFAULT_SOURCE =
   (
      SELECT Q2.RDB$DEFAULT_SOURCE
      FROM RDB$RELATION_FIELDS Q2
      WHERE (Q2.RDB$RELATION_NAME = 'TABLENAME') AND
            (Q2.RDB$FIELD_NAME = 'TMP_FIELDTOFIX')
   )
 WHERE (RDB$RELATION_NAME = 'TABLENAME') AND
       (RDB$FIELD_NAME = 'FIELDTOFIX')
;

-- ALTER TABLE DROP TMP_FIELDTOFIX;