Ind. Code § 6-9-26-15
(b) With respect to obligations for which a pledge has been made under subsection (a), the general assembly covenants with the holders of these obligations that:
(2) this chapter will not be amended in any manner that will change the purpose for which revenues from the tax imposed under this chapter may be used;
as long as the payment of any of those obligations is outstanding.
As added by P.L.74-1988, SEC.2. Amended by P.L.51-1992, SEC.4.