The investor, after written notice to the Secretary of his intention so to do, may terminate, as of the close of any operating year, any insurance contract made pursuant to this subchapter. The Secretary shall prescribe the events and conditions under which said Secretary shall have the option to terminate any insurance contract made pursuant to this subchapter, and the events and conditions under which said Secretary may reinstate any insurance contract terminated pursuant to this section or section 1747g(a) of this title. If any insurance contract is terminated pursuant to this section, the Secretary may require the investor to pay an adjusted premium charge in such amount as the Secretary determines to be equitable, but not in excess of the aggregate amount of the premium charges which such investor otherwise would have been required to pay if such insurance contract had not been so terminated.