If a majority of votes cast in the port district and, in addition thereto, a majority of the votes cast in the area to be annexed, favor such annexation, the board of county commissioners in each county in which any such land shall lie, shall enter an order declaring such port district enlarged so as to embrace within the limits thereof the territory described in the petition for such election, and thereupon the boundaries of said port district shall be so enlarged and the port commissioners thereof shall have jurisdiction over the whole of said district as enlarged to the same extent, and with like power and authority, as though the additional territory had been originally embraced within the boundaries of the existing port district; provided, however, that none of the lands or property embraced within the territory added to and incorporated within such port district shall be liable to assessment for the payment of any outstanding bonds, warrants or other indebtedness of the preexisting port district so enlarged, but such outstanding bonds, warrants or other indebtedness together with interest thereon, shall be paid exclusively from assessments levied and collected on the lands and property embraced within the boundaries of the preexisting port district.