The lawful use of a building or structure existing at the effective date of Ordinance 24-66, passed April 5, 1966 may be continued, although such use does not conform to the provisions hereof. If no structural alterations are made, a nonconforming use of a building may be changed to another nonconforming use of the same or of a more restricted classification. Whenever a nonconforming use has been changed to a more restrictive use or to a conforming use, such use shall not thereafter be changed to a less restricted use. The nonconforming use of a building may be hereafter extended throughout those parts of a building which were manifestly arranged or designed for such use at the time of the enactment of Ordinance 24-66, passed April 5, 1966. (Ord. 24-66. Passed 4-5-66.)