Notwithstanding the above, nothing contained in this section shall prevent the Board of Trustees from modifying a building permit, permit of compliance or conditional permit relating to the use of a structure, building or premises in the Residence E District as a multiple dwelling if, prior to the effective date of this section, such structure, building or premises had in effect a valid building permit, permit of compliance or conditional permit authorizing its use as a multiple dwelling, except that no such modification of an existing permit shall result in a more intensive use of the building, structure or premises.