No outdoor sign or display structure erected prior to the effective date of this section shall be rebuilt or relocated except in conformity with the provisions of this chapter. The requirements of this subsection shall not, however, be interpreted to prevent repairing or restoring to a safe condition any of the structural supports or the performance of maintenance operations on any existing sign, and, further, at such time that such maintenance is performed, no change in the existing sign shall be accomplished to make the sign nonconforming or nonconforming to any greater degree than at the time of adoption of this section.