When the contribution of a separately owned tract with respect to any unit pool has been established pursuant to subdivision (3) of Section 9-17-83, such contribution shall not be subsequently altered, unless the board shall find, after notice and hearing, that such contribution was erroneous because shown to be erroneous by subsequently discovered data or by subsequently discovered errors in the data upon which the original contribution was established. No change or correction of the contribution of any separately owned tract shall be given retroactive effect; provided, that appropriate adjustment shall be made for the investment charges as provided for in subdivision (4) of Section 9-17-83.