Any member (active or retired) may from time to time designate a new beneficiary or beneficiaries to receive payments due under N.H. Rev. Stat. § 102:21. Every such designation of a new beneficiary must be made in writing to the secretary of the retirement board, and such change shall not take effect until received by the retirement board. After receipt of such change, the change shall relate back to and take effect as of the date thereof, whether or not the member be living at the time of its receipt by the retirement board, but without prejudice to the board on account of any payments made by it before receipt of such written change. If the beneficiary shall die before the member, the interest of such beneficiary shall vest in the estate of the member, unless otherwise provided in the written directions of the member to the board. If no beneficiary be named by a member, any payments due under N.H. Rev. Stat. § 102:21 shall vest in the estate of such member. Payments which are due a beneficiary under legal disabilities may be made to the legal guardian or conservator of a minor or incompetent beneficiary or to the parents or surviving parent as natural guardian of a minor for whom no legal guardian has been appointed; and such payments shall constitute a final settlement of all claims on account thereof and shall bar recovery thereof by any other person or persons.