A nonconforming use of a portion of a building or structure, which building or structure otherwise conforms to the provisions of this Ordinance, shall not be expanded or extended into any other portion of such conforming building or structure, nor changed except to a conforming use. If the nonconforming use or portion thereof is discontinued or changed to a conforming use, any future use of such building, structure or portion thereof shall be in conformity to the regulations of the district in which such building or structure is located.