An incorporated city or town so adopting an official map by action of its city council or town commissioners may, whenever and as often as it may deem it for the public interest, change or add to such map, so as to place thereon lines and notations showing existing or proposed locations not theretofore mapped of new or widened public ways and new or enlarged parks and proposed discontinuances in whole or in part of existing or mapped public ways and parks. No such change or addition shall become effective until after a public hearing in relation thereto before the city council or town commissioners, at which parties in interest shall have an opportunity to be heard. At least 10 days' notice of such a public hearing shall be given by advertisement in a newspaper of general circulation in the city or town or in the county in which the city or town is located. No such change or addition which has not been previously recommended by the planning commission established by this chapter shall be adopted until after a report thereon by said commission and no variance from a plan prepared or approved by said planning commission shall be made except by a 2/3 vote of all the members of a city council or by a 2/3 vote of the town commissioners; provided, that the last mentioned requirement shall be deemed to be waived in case the matter has been referred to said commission for a report and it has failed to report within 30 days thereafter.