Any lawful sign existing on or before May 15, 1974, although such sign does not conform with the provisions of this chapter, may be continued as a nonconforming sign, except as provided herein, but if any such sign is changed, altered, modified, or its use discontinued, any further use of said sign must be in conformity with the provisions of this chapter. In addition, all nonconforming signs existing on or before May 15, 1974, shall be made to conform with the provisions of this chapter within ten years of the effective date of this section or be removed at the owner's expense. All nonconforming signs are subject to the provisions of §31-5 which requires a permit to erect, re-erect, remove, or maintain signs within the City; and the provisions of §31-8 which requires all signs to be kept in good repair and a safe condition.