The lawful use of any building or the use of land existing at the time of the adoption of this chapter may be continued, although such use or building may not conform with the provisions of this chapter, except as hereinafter provided.

A nonconforming use may not be changed to a more intensive nonconforming use, nor shall a conforming use be changed to a nonconforming use. Any nonconforming use, when changed to a conforming use, shall not thereafter be changed back to a nonconforming use.