Except as hereinafter provided, this zoning bylaw shall not apply to structures or uses lawfully in existence or lawfully begun, or to a building or special permit issued before the first publication of notice of the public hearing on this bylaw, but shall apply to any reconstruction, extension, or structural change of such structure and to any alteration of a structure begun after the first notice of said public hearing to provide for its use for a substantially different purpose or for the same purpose in a substantially different manner or to a substantially greater extent except where alteration, reconstruction, extension, or structural change to a single nonconforming nature of said structure.[1]