Amortization.
All signs rendered nonconforming by the provisions of this Article shall be altered, removed or otherwise made to comply with the provisions of this Article within the following time periods, which shall run from either the effective date of this Article or such later date as the use is made nonconforming:
Provided, however, that the following time periods shall apply to signs legally erected, pursuant to a valid sign permit issued within the two years immediately preceding the effective date of this Article: