WITH t AS
(
SELECT Null seq FROM dual WHERE 1=2
--UNION ALL SELECT '201310250000' FROM dual
UNION ALL SELECT '201310259999' FROM dual
--UNION ALL SELECT '20131025A123' FROM dual
--UNION ALL SELECT '20131025A999' FROM dual
)
SELECT TO_CHAR(sysdate, 'yyyymmdd')
    || CASE WHEN s = '9999' THEN 'A'
            WHEN SUBSTR(s, 2) = '999' THEN CHR(ASCII(SUBSTR(s, 1, 1)) + 1)
            ELSE SUBSTR(s, 1, 1)
        END
    || LPAD(MOD(SUBSTR(s, 2) + 1, 1000), 3, '0')
    AS new_seq
  FROM (SELECT NVL(SUBSTR(MAX(seq), 9), '0000') s
          FROM t
         WHERE seq LIKE TO_CHAR(sysdate, 'yyyymmdd')||'%'
        )
;

WITH t AS
(
SELECT Null seq FROM dual WHERE 1=2
UNION ALL SELECT '20131025YYYZ' FROM dual
--UNION ALL SELECT '20131025YYZZ' FROM dual
--UNION ALL SELECT '20131025YZZZ' FROM dual
)
SELECT s, x
     , RPAD(SUBSTR(s, 1, x)
       || REPLACE(CHR(ASCII(SUBSTR(s, x+1, 1))+1), ':', 'A')
       , 4, '0')
  FROM (SELECT s
             , LENGTH(RTRIM(s, 'Z')) - 1 x
          FROM (SELECT NVL(SUBSTR(MAX(seq), 9), '0000') s
                  FROM t
                 WHERE seq LIKE TO_CHAR(sysdate, 'yyyymmdd')||'%'
                )
        )
;