The lawful use of land existing on April 1, 1925, although such use does not conform to the provisions hereof, may be continued, but if such nonconforming use is abandoned, any future use of said premises shall be in conformity with the provisions of sections 14-401 to 14-418. The lawful use of a building existing on April 1, 1925, may be continued, although such use does not conform with the provisions hereof, and such use may be extended throughout the building, provided no structural alterations, except those required by law or ordinance are made therein. If no structural alterations are made, a nonconforming use of a building may be changed to another nonconforming use of the same or a higher classification. Whenever a use district shall be changed, any then existing nonconforming use in such changed district may be continued or changed to a use permitted in that district, provided all other regulations governing the new use are complied with. Whenever a nonconforming use of a building 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.