Mich. Comp. Laws § 113.14: Reincorporation from fourth to third class cities.

Sec. 14.

Whenever any city of the fourth class shall have a population of more than 10,000 according to the last preceding federal or state census, then such city shall be reincorporated as a city of the third class at the time and in the manner provided by law for the incorporation of cities of the third class.