Ind. Code § 8-1.5-5-26
With respect to bonds for which a pledge has been made under section 25 of this chapter, the general assembly covenants with the holders of these bonds that:
(2) this chapter will not be amended in a manner that will change the purpose for which revenues from the user fees imposed under this chapter may be used;
as long as those bonds are outstanding.
As added by P.L.125-1987, SEC.1.