Nothing herein shall require any change in the plans, construction or designated use of a building actually under construction at the time of the passage of Ordinance Number 486 or applicable amendments and which entire building shall be completed within two (2) years from the date of the passage of such ordinance. 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 building shall be completed within two (2) years from the date of the passage of such ordinance. 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 Ordinance Number 486 or applicable amendments shall apply to buildings or premises existing or buildings under construction or building permits issued in the area affected by such an amendment, at the time of the passage of such amendment.