If change in the method of election of school directors is approved at an election, the directors who were serving unexpired terms or were elected concurrently with approval of the change of method shall serve out the terms for which they were elected. If the plan adopted is that described in section 275.12, subsection 2, paragraph “b”, “c”, “d”, or “e”, the board shall at the earliest practicable time designate the districts from which residents are to be elected as school directors at each of the next two succeeding regular school elections, arranging so far as possible for elections of directors as residents of the respective districts to coincide with the expiration of terms of incumbent members residing in those districts. If an increase in the size of the board from five to seven members is approved concurrently with the change in method of election of directors, the board shall make the necessary adjustment in the manner prescribed in section 275.37, as well as providing for implementation of the districting plan under this section.