No existing building or premises devoted to a use not permitted by this chapter in the district in which such building or premises is located, except when required to do so by law or order, shall be enlarged, extended, reconstructed, substituted, or structurally altered to increase its nonconformity unless the use thereof is changed to a use permitted in the district in which such building or premises is located, and except as follows:
(a) Substitution. When authorized by the Planning and Zoning Commission, in accordance with the provisions of Section 1109.03, for the substitution of a nonconforming use, a nonconforming use may be replaced by another not more objectionable nonconforming use.
(b) Nonconforming Use Made to Conform. Whenever a nonconforming use has been changed to a conforming use, such use shall not thereafter be changed to a nonconforming use.