For comparison, WI's classes are:
1) Cities shall be divided into 4 classes for administration and the exercise of corporate powers as follows:
62.05(1)(a)
(a) Cities of 150,000 population and over shall constitute 1st class cities.
62.05(1)(b)
(b) Cities of 39,000 and less than 150,000 population shall constitute 2nd class cities.
62.05(1)(c)
(c) Cities of 10,000 and less than 39,000 population shall constitute 3rd class cities.
62.05(1)(d)
(d) Cities of less than 10,000 population shall constitute 4th class cities.
Like Mike D, you're not obligated to change classes when eligible, but powers change, esp. between 4th and 3rd.
Milwaukee is the only city in the state exempt from the class rankings. It operates under a special charter.