UPDATE RDB$ROLES SET RDB$OWNER_NAME='NEW_USER' WHERE RDB$OWNER_NAME='OLD_USER';

EXECUTE BLOCK
AS
  DECLARE VARIABLE tablename VARCHAR(32);
BEGIN
  FOR SELECT rdb$relation_name
  FROM rdb$relations
  WHERE rdb$view_blr IS NULL
  AND (rdb$system_flag IS NULL OR rdb$system_flag = 0)
  INTO :tablename DO
  BEGIN
    EXECUTE STATEMENT ('GRANT SELECT ON TABLE ' || :tablename || ' TO USER USER1');
    EXECUTE STATEMENT ('GRANT UPDATE ON TABLE ' || :tablename || ' TO USER USER1');
    EXECUTE STATEMENT ('GRANT INSERT ON TABLE ' || :tablename || ' TO USER USER1');
    EXECUTE STATEMENT ('GRANT DELETE ON TABLE ' || :tablename || ' TO USER USER1');
    EXECUTE STATEMENT ('GRANT REFERENCE ON TABLE ' || :tablename || ' TO USER USER1');
  END
END