Any association incorporated under the laws of this state for the purpose of furnishing insurance as provided for in this chapter is authorized to do business in the county in which its principal place of business is located, the counties contiguous thereto, and the next tier of contiguous counties and in other states where they are legalized and authorized to do business. Each association seeking to modify its authorized writing territory shall file with the commissioner a plan for controlled expansion demonstrating that provisions have been made adequately to service and protect policyholders. The expansion plan shall not be modified without the prior written approval of the commissioner, which approval shall not be unreasonably withheld. The words “mutual” and “association” shall be incorporated in and become a part of their name.