The lawful use of a building or parcel of land existing at the time of the adoption of this Zoning Ordinance may be continued, although such use of a building or parcel of land does not conform to the provisions hereof, provided no structural alterations are made other than those ordered by an authorized public officer to assure the safety of the building or structure and provided further, that such extension does not displace any residence use in a residence district. A non-conforming use may be extended, changed, or enlarged in accordance with the provisions found in this chapter.