Section 620. (a) Whenever the electors of a city adopt any of the optional charter plans provided by this act at any election for that purpose, such city shall be governed under the provisions of such charter plan and this act from the first Monday in January following the next succeeding municipal election.
(b) Whenever the electors of a city vote to discontinue such optional plan at any election for that purpose, the city shall cease to be governed by the provisions of such charter plan and this act on the first Monday of January following the next succeeding municipal election.