If the commissioners find that the proposed district, as described in the petition filed, will not embrace all of the lands that will be benefited by the proposed work, or that it will include lands that will not be benefited and not necessary to be included in said district for any purpose, they may extend or contract the boundaries of the proposed district so as to include or exclude all such lands, as the case may be; and the boundaries adopted and reported by them may, upon the hearing of their report as hereinafter provided, upon their application or that of any person interested, be altered by the court in such manner as shall appear to be just: provided, the alteration of boundaries as aforesaid shall not have the effect of so far enlarging or contracting the proposed district as to render such petition void or dismissable.