An insurer shall mail or deliver to the named insured at the mailing address shown on the policy written notice of premium increase, change in deductible or reduction in limits or substantial reduction in coverage at least thirty days before the expiration date of the policy. If the insurer fails to provide the thirty days’ notice, the coverage provided to the named insured remains in effect until notice is given or until the effective date of replacement coverage obtained by the named insured, whichever occurs first.