This chapter shall not supersede provisions of the Fire Prevention Code promulgated by the Board under § 27-97, that prescribe standards to be complied with in existing buildings or structures, provided that such regulations shall not impose requirements that are more restrictive than those of the Uniform Statewide Building Code under which the buildings or structures were constructed. Subsequent alteration, enlargement, rehabilitation, repair, or conversion of the occupancy classification of such buildings and structures shall be subject to the construction and rehabilitation provisions of the Building Code.