(a) When a district incurs a debt or obligation, it shall provide for payment of the debt or obligation by levying, assessing, and collecting either a general ad valorem tax or a tax on a benefit basis.
(b) Any district which has previously issued bonds or obligations payable on either basis may adopt a different basis of taxation in the creation of an additional debt or obligation.
(c) Each debt or obligation shall be paid in the manner provided at the time it was incurred.