Structures and their uses that are nonconforming may be continued so long as they otherwise remain lawful but shall not be enlarged, expanded or altered except to become more in conformity. In any non-residential zone, any replacement of a nonconforming principal structure shall conform to this chapter as follows. Any nonconforming principal structure destroyed to an extent greater than 50% of its assessed valuation shall not be reconstructed except in conformity with the zone requirements it is located, and accessory structures shall comply with the requirements provided for by § 126-361 below. If located in a floodway, any nonconforming use or structure partially damaged by fire, flood, casualty or act of God shall be terminated. Changes from one nonconforming use to another is prohibited.