Mich. Comp. Laws § 280.443
When in any consolidated district it shall be necessary to construct a new drain on or near the line or any part of the line of an existing, operating drain so as to make the old existing drain in whole or in part unnecessary, recognition and credit shall be given for such existing drain or part thereof as follows:
The assessing authorities shall make a determination of the value of benefits which, except for the construction of the new drain, could reasonably have been anticipated from the continued operation of the existing drain or part thereof.
The property or municipal corporation which paid the assessment for the existing drain or part thereof shall be exempt from any special assessment to pay for the new drain to the extent of the value so determined.
The assessing authorities, in assessing for benefits and apportionment of costs and expenses for any new drain in a consolidated district which has been laid on or near the line or any part of the line of an existing, operating drain so as to make the existing drain in whole or in part unnecessary, shall give effect to this exemption.
MCL 280.443