Any thrift institution other than a mutual bank may, with the approval of the department, convert itself into a mutual bank upon the affirmative vote of not less than a majority of the votes cast by those entitled to vote upon its affairs at a duly called and held meeting and shall thereupon possess the powers of and be subject to the duties imposed upon mutual banks under the provisions of this chapter. Before approving a conversion, the department shall determine that the thrift institution seeking conversion has the ability to discharge the duties and conform to the restrictions upon mutual banks and has previously so conformed to the extent required by the department. However, the institution may retain and service all accounts lawfully held by it on the date of its conversion.