Utah Code § 17-16-17
If the taxable value of any existing county has been reduced below or raised above the class and rank first assumed, the county legislative body of the county shall designate the class to which the county has been reduced or raised, and the county is in that class, and the salaries of county officers shall be adjusted on or before January 1 next succeeding by the county legislative body, but in no event may the salaries be reduced for the term for which the officers were elected and are qualified. The county legislative body in a newly created county shall at its first meeting after the organization of the county, for the purpose of fixing salaries and compensation of county officers, determine to which class the county belongs, and fix the salaries for the first term of the officers accordingly.
Utah Code § 17-16-17