Any contract of group disability insurance may provide for the readjustment of the rate of premium based on the experience thereunder at the end of the first year or of any subsequent year of insurance thereunder, and such readjustment may be made retroactive only for such policy year. Any refund under any plan for readjustment of the rate of premium based on the experience under group policies heretofore or hereafter issued, and any dividend paid under such policies may be used to reduce the employer's share of the cost of the coverage, except that if the aggregate refunds or dividends under such group policy and any other group policy or contract issued to the policyholder exceed the aggregate contributions of the employer toward the cost of the coverages, such excess shall be applied by the policyholder for the sole benefit of insured employees.