The lawful use of a building or lot existing at the time of the adoption of this Chapter or, as applicable, of the adoption of any amendments to this Chapter may be continued as lawful even though such use does not conform with the provisions of this Chapter or applicable amendments thereto and provided that lawful non-conforming industrial or commercial uses may be enlarged up to fifty percent (50%) of their building area existing on the effective date of this Chapter or, as applicable, of the adoption of any amendments to this Chapter. If no structural alterations are made, a non-conforming use of a building may be changed to another non-conforming use of the same or more restrictive use classification. Whenever a non-conforming use of a building has been changed to a more restrictive use or to a conforming use, such changed use shall not thereafter be changed to a less restrictive use.