This bylaw shall not apply to existing buildings or structures, nor to the existing use of any building or structure or of land, to the extent to which it is legally used at the time of adoption of this bylaw, but it shall apply to any change of use thereof and to any alteration of a building or structure when the same would amount to reconstruction, extension or structural change, and to any alteration of a building or structure to provide for its use for a purpose or in a manner substantially different from the use to which it was put before alteration, or for its use for the same purpose to a substantially greater extent.

In cases of mixed occupancy, the regulation for each use shall apply to the portion of the building or land so used.