If any school district hereafter becomes a district of the first class, a new board of school directors shall be appointed in the manner provided in this act for the appointment of school directors in districts of the first class. If any school district hereafter becomes a district of any of the other classes and the number of directors therein is increased, the additional directors shall be appointed or elected as vacancies in the board of directors are now filled and for terms corresponding to those of directors in other school districts of the same class. When the change reduces the number of directors in such district, the school directors then in office shall hold office during the respective terms for which they are elected, and as vacancies are about to happen by the expiration of the terms of any directors, a number of directors shall be elected at the municipal election preceding such vacancies equal to the number of directors required by law to be elected at the corresponding election in other school districts of the same class.