Continuance. Any nonconforming use of land, buildings or structures lawfully existing at the effective date of this chapter may be continued, and any building so existing which was designed, arranged, intended for or is devoted to a nonconforming use may be structurally altered and the nonconforming use therein continued, subject to the conditions set forth below. Similarly, whenever a district shall be changed hereafter, the provisions of this chapter with regard to any building or uses lawfully existing at the time of the passage of this chapter shall apply, subject to the conditions set forth below, to any building or use lawfully existing in such changed district at the time of the passage of such amendment.