Nothing, herein contained, shall require any change in the plans, construction or designated use of a building actually under construction, at the time of the passage of this ordinance, and which entire building shall be complete within one year from the date of the passage of this ordinance. If any amendment to this ordinance is hereafter adopted changing the boundaries of the districts, the provisions of this ordinance, with regard to building or premises existing or buildings under construction, or building permits issued at the time of the passage of this ordinance, shall apply to buildings or premises existing, or building under construction, or building permits issued in the area affected by such amendment at the time of the passage of such amendment.