Termination by Discontinuance or by Substitution. If any nonconforming use, as defined in Subsection (b) of this section, is closed, vacated, abandoned, or not operated for a period of six months, any legal permit in effect at the end of such six month period shall become null and void and the status of the premises as a legal nonconforming use shall be terminated. A substitution or change of the use of the premises shall constitute a vacated or abandoned use.