Between January 1 and March 30 of each year, the secretary of the district shall inspect the assessable area within the district. At such time as at least 50 percent of the assessable area within the district is devoted to and developed for residential, industrial, or nonagricultural commercial use, or any combination thereof, such fact shall be certified to the board of directors by the secretary of the district. Any time after such certification the registered voters residing within the district may petition for a change in the voting procedure from a landowner voting district to a resident voting district.