Textbook answer in common law regarding changes to contractual obligations forced by a change in statutory law:
A contract is discharged when its performance is made illegal by a subsequent change in the law. For example, suppose there is a contract to construct a three story building at a particular place. Prior to beginning construction, a zoning law is passed which prohibits such a building in this area. The contract would be discharged. However, a change of law that merely increases the cost of one of the parties is not a “change of law” that discharges the contract. For example, Mack made a contract with Richard to build an apartment house for a specific price. A number of serious apartment house fires later occurred in the city, and an ordinance was adopted by the city council increasing the fire precautions that had to be taken in the construction of a new building. Compliance with these new requirements would make the construction of the apartment house for Richard more expensive than Mack had originally contemplated. Is Mack discharged from the contract to build the apartment house? No. Mack must perform the contract even though it has been made more expensive to do so because of the change of law. Mack is not discharged from the contract, because the change of law did not prohibit the construction of the building but merely made it more expensive to build. - See more at:
Discharge and Substitution of a Contract - Discharge ? Performance - Contracts