In case of review of a conditional use request where there is an existing nonconforming use of land under Article XI hereof, the Planning Board may impose such reasonable conditions, including, but not limited to, the placing of fencing and screening, as will minimize the impact such open use has upon surrounding residential properties. In such cases the Planning Board may also permit reasonable changes in existing structures on the land, within the limitations of the district in which the use is located, for the purpose of limiting the open use of the land.