The terms of office of trustees on the board for the state school for the blind who are appointed from the various congressional districts shall not be affected by the creation of either new boundaries for congressional districts or additional districts. In such an event, each trustee may continue to serve in office for the balance of the term for which he or she was appointed: PROVIDED, That the trustee continues to reside within the boundaries of the congressional district as they existed at the time of his or her appointment. Vacancies which occur in a trustee position during the balance of any such term shall be filled pursuant to RCW 72.41.020, as now or hereafter amended, by a successor who resides within the boundaries of the congressional district from which the member whose office was vacated was appointed as they existed at the time of his or her election. At the completion of such term, and thereafter, a successor shall be appointed from the congressional district which corresponds in number with the congressional district from which the incumbent was appointed.