Once a nonconforming use has been changed to a conforming use or a nonconforming structure has been altered so as to comply with the yard, height, parking, loading, and access provisions of this chapter, it shall not revert back to a nonconforming use or structure. Once the Zoning Board of Appeals has permitted the substitution of a more restrictive nonconforming use for an existing nonconforming use, the prior existing use shall lose its status as a legal nonconforming use and the substituted use shall become subject to all the conditions required by the Board.