All fences, walls and windbreaks which exist on December 19, 1966 (the time of adoption of the original ordinance affecting fences, walls and windbreaks), and do not conform to the chapter, may be maintained, but no alteration, modification or improvement of such nonconforming fence, wall or windbreak shall be permitted unless, as a result of such alteration, modification or improvement, such nonconforming use shall be made to comply with this chapter.