Except as otherwise provided herein, the lawful use of a building existing at the effective date of this Zoning Ordinance may be continued and expanded by an amount not to exceed two times its present floor area, 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 restricted 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 lawfully and manifestly arranged or designed for such use at the time of the enactment of this Ordinance. (Ord. 1973-9. Passed 12-17-73.)