And that PERS case cited the US Constitution, not just the Oregon Constitution.

"We hold that Sections 10, 11, and 12 of Article IX substantially impair the state's contractual obligations to plaintiffs in violation of the federal Contracts Clause and that the impairment is not justified by any significant and legitimate public purpose. Accordingly, we declare Sections 10, 11, and 12 of Article IX (Ballot Measure 8) void."


http://law.justia.com/cases/oregon/supreme-court/1996/323-or-356.html