The lawful use of a sign existing at the time of the effective date hereof may be continued even though such use may not conform to the regulations herein. For those signs permitted before the adoption of these regulations, such signs shall be classified as “permitted nonconforming structures.” No sign permit shall be issued for any lot, tenant, or development after the effective date hereof and not in substantial conformity with the provisions of this chapter. Nor shall any sign, except as hereinafter specified, be erected, substantially improved, converted, enlarged, moved, or structurally altered without conforming with the provisions of this chapter.