Except as otherwise provided in this section, the lawful use of land or buildings existing October 22, 1969, may be continued, although such use or building does not conform to the regulation specified by this chapter for the zone in which such land or building is located; provided, however, that:

A use shall be adjudged as abandoned when there occurs a cessation of any use or activity by an apparent act or failure to act on the part of the tenant or owners to reinstate or change such use.

Any use or structure validly existing under the law on October 22, 1969, which does not conform with the provisions of this chapter may be continued upon the lot or in the building so occupied or any such structure may be restored or prepared, but not enlarged, in the event of partial destruction. The use, matter or purpose may be continued, notwithstanding any change of title or right to possession. A nonconforming use may not be changed to another nonconforming use. A nonconforming use which is abandoned may not be reestablished.

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

A nonconforming building may not be reconstructed or structurally altered during its life to an extent exceeding in aggregate cost 50% of the assessed value of the building unless such building is changed to a conforming use.

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 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, which has work of which has 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 October 22, 1969.

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