After any such agreement has been recorded as provided in NRS 539.595, no subsequent board of directors shall have any authority to repeal, change or modify the same in any respect whatsoever without the consent in writing of the owners and holders of all of the bonds of the district then outstanding; but the board of directors may, in its discretion, increase the amount of such sinking fund tax, or levy additional sinking fund taxes, or other taxes, from time to time without in any wise impairing the obligation to levy and collect such minimum sinking fund tax.
NRS 539.600