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