--INPUT 
--777XX REQ MSD 20022019 3

--OUTPUT
--777XX RES MSD 16082019 1

SELECT
    replace(regexp_substr('777XXREQMSD200220193', '^\d{3}[A-Z]+'), 'REQ', 'RES')
    || TO_CHAR(SYSDATE, 'DDMMYYYY')||'1'
FROM
    dual;

SQL> select regexp_replace(
  2           '777XXREQMSD200220193',
  3           '^(\d{3}[A-Z]*)REQ([A-Z]*)\d+$',
  4           '\1RES\2'||TO_CHAR(SYSDATE, 'DDMMYYYY')||'1') res
  5  from dual
  6  /
RES
---------------------------------------------------------------------
777XXRESMSD190820191

1 row selected.