Nonconforming signs may be maintained until such time as they are damaged, destroyed, removed, or in need of repair to a degree of 50% or more of value of the sign. At such time, the sign may only be repaired or replaced in strict conformance with this chapter. A nonconforming sign, shall not be expanded, enlarged, or structurally changed. Changes to the facing of the sign only, with no change to the physical structure of the sign, are permissible.