As soon as practicable after January 1, 1985, the supreme court administrator shall recompute the number of judgeships to which judicial election districts 5A and 5C are entitled. Notwithstanding section 602.6201, subsection 2, the seventeen incumbent district judges in judicial election district 5A on December 31, 1984, may reside in either judicial election district 5A or 5C beginning January 1, 1985. The supreme court administrator shall apportion to judicial election district 5C those incumbent district judges who were appointed to replace district judges residing in Polk county or who were appointed to fill newly created judgeships while residing in Polk county. The incumbent district judges residing in Polk county on January 1, 1985, who are not so apportioned to judicial election district 5C shall be apportioned to judicial election district 5A but shall be reapportioned to judicial election district 5C, in the order of their seniority as district judges, as soon as the first vacancies occur in judicial election district 5C due to death, resignation, retirement, removal, or failure of retention. Such a reapportionment constitutes a vacancy in judicial election district 5A for purposes of section 602.6201. Notwithstanding section 602.6201, subsection 2, the seventeen incumbent district judges in judicial election district 5A on December 31, 1984, shall stand for retention in the judicial election district to which the district judges are apportioned or reapportioned under this section. Commencing on January 1, 1985, vacancies within judicial election districts 5A and 5C shall be determined and filled under section 602.6201, subsections 4 through 8. For purposes of the recomputations, the supreme court administrator shall determine the average case filings for the latest available three-year period by reallocating the actual case filings during the three-year period to judicial election districts 5A and 5C as if they existed throughout the three-year period.