In legal circles, contracts and promises can be construed differently.  The PERA benefit is in statute and is not constitutional, whereas most other states have their pension obligations enshrined or guaranteed in their respective state constitutions.  Statutes can be changed by succeeding legislatures. 

In 2015, the Colorado Supreme Court ruled that post-retirement benefits can be changed to serve a greater public purpose or in case of actuarial emergency. A greater public purpose could include expanding Medicaid to cover a quarter or a third of Coloradans. An actuarial emergency can come about by public employers not making actuarial required contributions (ARC) for the last 15 years (as with Colorado). Indeed, Colorado has expanded Medicaid in percentage terms (or per capita terms) more than all 50 states, except Kentucky which is number one. The Colorado governor is asking PERA retirees and members to keep taking hits to help support state spending priorities.