Nonconforming signs may not be enlarged, extended, relocated or altered in any way, except to make them conform to provisions of this chapter. This provision shall not restrict routine maintenance of nonconforming signs involving replacement of electrical parts and repainting. Ordinary maintenance and repairs may be made to any nonconforming sign, provided that the structure, advertising display matter, lettering, color scheme or accessories are not altered, modified, changed, reconstructed or moved, and provided that such ordinary maintenance and repairs do not exceed 20% of the value of the sign in any one-year period.