Any nonconforming use of land or structures may be continued for definite period of time subject to such regulations as the city may require for immediate preservation of the adjoining property. The building inspector may grant a change of occupancy from one nonconforming use to another, providing the use is within the same, or higher or more restricted classification as the original nonconforming use. In event a nonconforming use of a building may be changed to another nonconforming use of more restricted classification, it shall not later be changed to a less restrictive classification (but) shall be considered to have been abandoned.