Ariz. Rev. Stat. § 48-817
A fire district with a population exceeding twenty-five thousand persons as estimated by the board of supervisors shall not organize or reorganize as a fire district administered by an elected chief. If the board of supervisors, at any time prior to one hundred twenty days before the next regularly scheduled election for the district, determines that the estimated population of a fire district administered by an elected chief exceeds twenty-five thousand persons, the board of supervisors shall order the reorganization of the fire district as a district administered by a district board effective upon the election of five members of the district board. The estimates of population shall be as provided in section 48-803, subsection A.
A.R.S. § 48-817