If a lawful use of a structure, or of structure and premises in combination, exists at the effective date of adoption or amendment of this Zoning Ordinance that would not be allowed in the district under the terms of this Ordinance, the lawful use may be continued so long as it remains otherwise lawful, subject to the following provisions:
(a) No existing structure devoted to a use not permitted by this Zoning Ordinance in the district in which it is located shall be enlarged, extended, constructed, reconstructed, moved or structurally altered except in changing the use of the structure to a use permitted in the district in which it is located.
(b) Any nonconforming use may be extended throughout any parts of a building which were manifestly arranged or designed for such use at the time of adoption or amendment of this Zoning Ordinance, but no such use shall be extended to occupy any land outside such building.
(c) If no structural alterations are made, any nonconforming use of a structure, or structure and premises, may be changed to another nonconforming use provided that the Board of Appeals, either by general rule or by making findings in the specific case, finds that the proposed use is equally appropriate or more appropriate to the district than the existing nonconforming use. In permitting such change, the Board of Appeals may require appropriate conditions and safeguards in accordance with the provisions of this Zoning Ordinance.