Within 10 days after the withdrawal of an insurer from the plan or from the time additional insurers participate in the plan, notice of the change shall be given the commissioner in writing, and unless such notice is so given, the permit theretofore issued by the commissioner shall expire on the tenth day after such change. Unless the permit expires as provided in the preceding sentence, it shall remain in force continuously unless the commissioner after hearing finds that the insurers have deviated materially from the plan, and that it is against public interest to continue its operation.