At any time during the term of the contract for the furnishing of artificial lights under section 515.08 of the Revised Code, but not more than once in each year, unless mutually satisfactory to both parties to the contract, the board of township trustees may increase the number of lights to be maintained under the contract, at not more than the same price per light, and the contract shall provide for such change in the number of lights.
R.C. § 515.09