If at the conclusion of the hearing the legislative body by resolution adopted by four-fifths of all of its members shall find and determine (a)that public interest and convenience require the substitution, and (b)that all of the lands within the district will be benefited by the substituted acquisition or acquisition and improvement in substantially the same degree and in substantially the same proportion as said lands were benefited by the old property, said legislative body may then order the substitution either as described in the resolution proposing to order the making of a substitution, or as subsequently modified after proceedings pursuant to Section 31924.