The board of directors may in the manner provided by this article for the conversion of district assessments, similarly convert into mortgage liens or contracts district taxes then levied and assessed, except taxes levied and assessed for payment of principal or interest on existing district bonds satisfaction of which is not then otherwise provided for, and may subordinate the liens so converted to any previous assessment lien converted into a realty mortgage lien or to a refunding bond issue. A district tax so converted may be fixed and established by the board as a lien prior and superior to any other lien or encumbrance then existing against the lands of the district except the preferred, prior and converted first mortgage liens, unsatisfied bond tax levies not then otherwise provided for, and state, county and district taxes thereafter assessed against such district lands.