As often as may be necessary, the retirement board shall cause to be made a thorough investigation of the several funds or account of the retirement system for the purpose of determining the rates at which the benefits will be granted. It shall make adjustments in such rates as, upon recommendation of the actuary, may appear to be proper for maintaining solvency of the several funds or account. No revision of rates shall affect adversely the rights of any beneficiary under an application made prior to such revision. The retirement board shall, from time to time, order and make such distribution of gains and savings to the several funds or account as it may deem equitable.