In any case involving insurance subject to this part 4 on which an insured has prepaid a premium for the issuance of a policy of insurance for a specified policy period, the insurer shall not increase unilaterally, during said policy period, the rate charged nor decrease the coverage benefits provided unless there is a change in risk during the policy term attributable to any act or acts of the insured or the risk to be insured was misrepresented by the insured. This section shall not prohibit cancellation of a policy for any reason otherwise permitted by the policy or by law during an initial policy period of not to exceed sixty days.