If there is more than one street lighting system in any city any or all of such systems may be consolidated and improved under one proceeding. In such case the consolidated system shall be zoned so that those systems of approximately the same general type are grouped together.

In any district wherein zones have been established under this part and where subsequent to such establishment, improvements are made to portions of the lighting system in any zone which are of the same general type as in other existing zones within the district, the portion of the zone which has been so improved may be transferred to a zone with a lighting system of the same general type pursuant to the same procedure of resolution, notice and hearing as are applicable under this chapter for the formation of the district.