In the event any district hereafter purchases or otherwise acquires electric utility properties comprising all or a portion of an electric generation and/or distribution system from a public service company, as defined in RCW 80.04.010, the total amount of privilege taxes imposed under chapter 278, Laws of 1957 to be paid by the district annually on the combined operating property within each county where such utility property is located, irrespective of any other basis of levy contained in this chapter, will be not less than the combined total of the ad valorem taxes, based on regular levies, last levied against the electric utility property constituting the system so purchased or acquired plus the taxes paid by the district for the same year on the revenues of other operating property in the same county under terms of this chapter. If all or any portion of the property so acquired is subsequently sold, or if rates charged to purchasers of electric energy are reduced, the amount of privilege tax required under this section shall be proportionately reduced.