All the assets of the retirement system shall be credited according to the purpose for which they are held to one of four funds; namely, the members’ savings fund, the benefit reserve fund, the general reserve fund, and the expense fund. The board of trustees is authorized to transfer and close out the fund accounts as provided prior to the effective date of sections 87.120 to 87.370 into the appropriate fund accounts provided by sections 87.120 to 87.370.