§ 13. Application of chapter to existing dwellings. Any building erected and occupied on or before April eighteenth, nineteen hundred twenty-nine, or thereafter, as a tenement, which is not recorded as such in the department, shall be required to comply with all the provisions governing dwellings of like class or kind erected after such date. Except as otherwise expressly required in this section and in sections nine and twenty-five, subdivision six of section thirty-one, and sections thirty-three, sixty-six and sixty-seven and in articles six and seven, nothing in this chapter shall be construed to require any change in the construction, use or occupancy of any multiple dwelling lawfully occupied as such on April eighteenth, nineteen hundred twenty-nine, under the provisions of all local laws, ordinances, rules and regulations applicable thereto on such date; but should the occupancy of any such dwelling be changed to any other kind or class after such date, such dwelling shall be required to comply with the provisions of section nine.