Applicability of section. The following regulations shall govern the erection and alteration of buildings and structures and the use of land in the Industrial District; except, however, that these regulations shall not apply to an existing building or structure, nor to the existing use of any building, structure or land, if such existing building or structure or existing use of any building, structure or land was a lawful use under the governing provisions of prior zoning ordinances, nor to an alteration thereto for a use or for any purpose or in any manner which is not substantially different from that to which it was put before alteration, nor to lots shown in a subdivision plat which has been approved by the Salem Planning Board. However, in the event of discontinuance of a nonconforming use for whatever reason for 12 consecutive months, then the following regulations shall govern the erection and alteration of buildings and structures and the use of land in the Industrial District. Further, signs of all types and descriptions must conform to all requirements of this chapter not later than April 1, 1982.