Congress had no contract with GM bond holders and did not interfere with any contract involving them.
There are several issues which would stop Congress from abrogating the contract it has with fed workers. Any such law would appear to be a violation of the Constitutional ban on ex post facto Laws.
Such a law would also be a form of confiscation of private property without compensation since the labor agreed to be provided was performed under an agreement as to compensation.
Marshall dealt with similar issues (Gibbons vs Ogden, Hunters Leasee, Yazoo) as long ago as 200 yrs. I have no doubt the courts would rule very quickly to stop any ex post facto changes in fed compensation.
Nothing is guaranteed, period.
Not welfare, not SS, not medicare, not pensions, nothing.
Congress can do whatever it wants, it can raise them, lower them, raise the retirement age for getting them, limit how much you can get from various government sources, cut them entirely.