If, at any time, any sign in existence or maintained on the effective date of § 25-47, which does not conform to the provisions of § 25-47, is destroyed by fire, accident, explosion or act of nature to the extent of more than 50% of the value thereof, such sign shall be subject to all the provisions of this § 25-47. For the purposes of this § 25-47, the value of any sign shall be the estimated cost of replacement of the sign in kind as determined by the Chief Building Official.