Section 223. Modification of board decisions or orders

Upon its own motion
  or upon the application of any party in interest, the board may  at  any
  time  review  any  decision  or  order  and,  on such review, may make a
  decision ending,  diminishing  or  increasing  the  benefits  previously
  ordered,  and  shall state the reason therefor.  Upon the filing of such
  decision, the chairman shall send to each of the parties a copy thereof.
  No such review shall affect any previous decision as regards any  moneys
  already  paid, except that a decision increasing the benefit rate may be
  made effective from date of commencement of disability, and except that,
  if any part of the benefits due is unpaid,  a  decision  decreasing  the
  benefit  rate may be made effective from the commencement of disability,
  and any payments made prior thereto in excess  of  such  decreased  rate
  shall be deducted from future benefits in such manner and by such method
  as may be determined by the board.