A sign existing on the effective date of adoption of this section[1] which does not conform to any provision thereof, shall be deemed a nonconforming use and may be continued, maintained, and repaired upon its present premises, provided that such sign was lawful under any prior ordinance. Any legal nonconforming sign on any premises involving a change in tenancy of said premises or involving a change in the physical appearance of the sign shall revoke the nonconforming rights as soon as the physical appearance of the sign is changed in any manner whatsoever. Nothing herein shall be construed to prohibit the normal maintenance of a legal, nonconforming sign, such as bulb replacement, painting, or replacement of existing lettering.