Every financial entitlement or benefit is based on law. Laws can be changed. So the short answer is yes, they could renig. This gives rise to the saying "Nothing is safe when Congress is in session." Would they? It would be political suicide.
I've been reading about how Stockton is filing bankruptcy and everyone that worked for the city that was drawing a pension and uses earned health benefits will not longer have those benefits. I understand the difference on the larger scale but I guess with he economy the way it is anything is possible!