If the agreement filed with the chairman under subdivision (b) of section 358-3.4 of this Subpart is the agreement of a carrier and not of the employer, acceptance of the plan shall terminate forthwith whenever the carrier's obligation to pay benefits shall terminate, unless prior thereto an acceptable new carrier, assuming fully the obligations of the employer under the plan, has duly filed with the chairman notice thereof and has filed also the agreements with the chairman required under section 358-3.4.