Existing facilities cannot engage in or employ a use, activity, or structure listed in prohibited uses, par. (d) above which they did not engage in or employ at the time of enactment of a district and can only expand, replace in kind or rebuild those present uses, activities, equipment, or structures on the site or property of record associated with the facility at the time of enactment of a district, and in a manner that improves the environmental and safety technologies already being utilized. No existing use, activity, or structure listed as prohibited use or conditional use shall be expanded, replaced in kind, or rebuilt unless a conditional use permit is granted for expansion, replacement, or rebuilding. This section does not apply to normal maintenance or minor repairs.