Preexisting nonconforming structures, other than single- or two-family structures, or uses may be extended or altered, provided that no such extension or alteration shall be permitted unless there is a finding by the Board of Appeals that such change, extension or alteration shall not be substantially more detrimental than the existing nonconforming structure or use to the neighborhood. This section shall not apply to billboards, signs and other advertising devices subject to the provisions of MGL C. 93, §§ 29 through 33 and to C. 93D or to establishments which display live nudity for their patrons, as defined in § 9A of MGL C. 40A, adult bookstores, adult motion-picture theaters, adult paraphernalia shops, or adult video stores subject to the provisions of § 9A.