When an insurance carrier has certified a motor vehicle liability policy under section 321A.19 or section 321A.20, the insurance so certified shall not be canceled or terminated until at least ten days after a notice of cancellation or termination of the insurance so certified shall be filed in the office of the department, except that such a policy subsequently procured and certified shall, on the effective date of its certification, terminate the insurance previously certified with respect to any motor vehicle designated in both certificates.