Except as otherwise provided in this article, the lawful use of land or buildings existing on June 30, 1969, may be continued, although such use or building does not conform to the regulations specified by this chapter for the zone in which such land or buildings are located; provided, however:

A nonconforming use shall terminate upon abandonment at any time. In the case of discontinuance of any nonconforming open land use for a period of 12 consecutive months, the nonconforming open land use shall be presumed to be abandoned and any subsequent use of the land shall conform to the regulations for the district in which such land is located. In addition, when a nonconforming use of a structure, or structure and premises in combination, is discontinued for a period of 24 consecutive months, the nonconforming use shall be presumed to be abandoned and the structure, or structure and premises in combination, as the case may be, shall not thereafter be used except in conformance with the regulations of the district in which it is located.

A structure which, by reason of restrictions on area, lot coverage, height, or yard, is nonconforming shall not be reconstructed except in conformance with this chapter should the structure be destroyed by any means to an extent of more than 75% of its replacement cost at the time of destruction. Nothing in this chapter shall prevent the strengthening or restoring to a safe condition of any wall, floor or roof which has been declared unsafe by the Construction Code Official.

No nonconforming use shall, if once changed into a conforming use, be changed back again into a nonconforming use.

A nonconforming building, structure or sign may be reconstructed but not enlarged or extended unless the building is changed to a building conforming or more nearly conforming to the requirements of this chapter.

Nothing herein contained shall require any change in plans, construction or designated use of a building, structure or sign for which a building permit has been heretofore issued and the construction of which shall have been diligently prosecuted within three months of the date of such permit, and the ground story framework of which, including the second tier of beams, shall have been completed within six months of the date of the permit, and which entire building shall be completed according to such plans as filed within one year from June 30, 1969.

Wherever the boundaries of a zone shall be changed as to transfer an area from one zone to another of a different classification, the foregoing provisions shall also apply to any nonconforming uses existing therein or created thereby.