Extra compensation may not be granted or allowed by the General Assembly to any public Officer, Agent, Servant or Contractor, after the service has been rendered, or the contract entered into; nor may the salary or compensation of any public officer be increased or diminished during his term of office except those whose full term of office is fixed by law in excess of 4 years. However, after January 1, 1956, for services rendered after that date, the salary or compensation of any appointed public officer of the Mayor and City Council of Baltimore may be increased or diminished at any time during his term of office; except that as to officers in the Classified City Service, when the salary of any appointed public officer of the Mayor and City Council of Baltimore however, increased or decreased, it may not again be increased or decreased, as the case may be, during the term of such public officer.