Nonconforming signs. Every lawful nonconforming sign shall be discontinued and removed or changed to a conforming sign within a period of five years from the effective date of this chapter; provided, however, that signs which, at the effective date of this chapter, are maintained in connection with and upon the same lot as a lawful nonconforming use may be continued, maintained, repaired or replaced with signs of similar size and character as long as such nonconforming use continues, but may not be enlarged or otherwise substantially altered, nor may the illumination or lack of illumination thereof be changed, except in accordance with the regulations applicable to the district in which it is located.