Should such structure be damaged by reason of fire, explosion, windstorm or the elements or by any other means (including, without limitation, any demolition, voluntary or otherwise) to an extent of either more than 50% of its replacement cost or a maximum of 50% of its cubic content at the time of damage, it shall not be reconstructed except in conformity with the provisions of this Part 1; provided, however, that where such destruction within Districts A and B occurs by means other than demolition, voluntarily or otherwise, such structure may be reconstructed within its existing foundation lines, provided that such reconstruction within existing foundation lines does not violate any existing front and/or rear yard depth requirements and that such construction is not in violation of a five-foot side yard width restriction in the uses permitted by right as set forth in A District restrictions and a four-foot side yard width restriction in the uses permitted by right as set forth in B Districts.