This will depend on the number of levels you want to go with this, but this should accomplish what you wanted:
CASE
when PATINDEX('%Z%',EmployeeCode) > 0 then REPLICATE('A',PATINDEX('%[0-9]%',EmployeeCode)) + SUBSTRING(EMPLOYEECODE,PATINDEX('%[0-9]%',EmployeeCode),99)
WHEN ISNUMERIC(LEFT(EMPLOYEECODE,1)) = 1 THEN 'A'+EMPLOYEECODE
ELSE CHAR(ASCII(LEFT(EMPLOYEECODE,1))+1)+SUBSTRING(EMPLOYEECODE,2,99)
END