If buildings erected for, or converted to, the uses of any classification mentioned in this chapter, are to be applied to the uses of any other class for which a better system of construction is required by this chapter, the construction and equipment of such buildings shall first be made to conform to the requirements of this chapter, as specified for their intended use. It shall be unlawful to use any such building for a new or different purpose from that to which its structure and equipment adapt it under this chapter, unless the requirements of this chapter and the requirements of chapter for such new or different use shall first have been complied with.