The pension board shall effect a division into two parts exclusive of the division required by Section 31891, for the purposes of this article, of the retirement system. One part shall consist of those who are active members of the retirement system on division date, and who do not desire coverage under the federal system; and the second part shall consist of those who are active members of the system on division date, and who desire coverage under the federal system and of those who become members of the system after division date. The division date shall be set by the pension board so as to permit modification of the federal-state agreement to include services of employees who are members of the second part of said system.