If a proposed charter for a consolidated city has been approved by the General Assembly for adoption in any area in which a consolidation of localities is proposed to be effected in accordance with the provisions of this article, then in any subsequent proceedings under the provisions of this article, such charter may be used as the basis for a new consolidation agreement, or upon petition of ten percent of the registered voters of any county and city as of January 1 of the year in which the petition is filed subject to the provisions of this article, such proposed charter may be submitted to the voters of such counties and cities for adoption as the charter of the consolidated city and shall in all respects fulfill the requirements of the consolidation agreement provided for in this article.