Except as otherwise provided, any use existing on December 16, 1963, 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 building is located; provided, however, that:

When a nonconforming use of land, structure and premises in combination is discontinued or abandoned for six consecutive months or for 18 months during any three-year period (except when government action impedes access to the premises) the land, structure or structure and premises in combination shall not thereafter be used except in conformity with the regulations of the district in which it is located.

Any nonconforming building which shall be more than 50% destroyed by reason of windstorm, fire, explosion or other act of God or the public enemy shall be deemed completely destroyed, and the structure may not be rebuilt, restored or repaired, except in conformity with the regulations of this chapter. 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 Building Subcode Official.

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

A nonconforming building may not be altered, enlarged or expanded during its life unless said building is changed to a building conforming to the requirements of this chapter.

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