Why would it be difficult? Are you thinking of ex post facto doctrine? That really only applies to criminal penalties. A good, long-term contract should require

Why would it be difficult? Are you thinking of ex post facto doctrine? That really only applies to criminal penalties. A good, long-term contract should require that if some part of it is invalidated by changes to law (or a court finds that it violates existing law), then the parties can or must renegotiate that part but the rest of the contract survives, or alternatively that the entire contract becomes void if that's what the parties want. There really isn't a way to include a clause in a contract that says the parties are free to ignore changes in laws.