. . " alter the power hereby vested in the district to establish and collect such fees and charges as will produce revenues sufficient to pay the items specified in subparagraphs (a), (b), (c), and (d) of this Section 8, or in any way to impair the rights or remedies of the holders of the bonds, or of any person in their behalf, until the bonds, together with the interest thereon, with interest on unpa" . . .