> We have a concept in US Law called “ex post facto” which means you are not liable after the fact if laws are changed.

That's not what ex post facto means. An ex post facto law is a law criminalizing an act or increasing criminal penalties after an offense occurred. It does not prevent retroactive civil liability.

The more applicable concepts here are the binding nature of contracts (which settlements are) and res judicata (which means that, in short, once a court judgement has been entered and made final, a party cannot relitigate it to get a better outcome.)