Ordinary repairs and nonstructural alterations may be made to a nonconforming building or structure. However, no structural alteration shall be made to a building or structure, all or substantially all of which is designed or intended for a use not permitted in the district in which it is located, except those required by law. For the purpose of this section, ordinary repairs and alterations shall be determined by the zoning administrator. "Structural alterations" are those which move, increase or otherwise modify any structural support for the building or structure. The replacement of storage tanks where the safety and operation of the installation requires such replacement shall also be permitted. Notwithstanding any of the above requirements, structural alterations may be made to any nonconforming building or structure in a manner so that the entire building or structure and use conform to all regulations of the district in which it is located. (1965 Code title 6 § 501.3)