There are many more facts that are needed to give an answer. Some of these are:
What does the contract say about treatment? Is the contract with the hospital or a payor (insurance company)? Who is the payor, Medicare?
I am licensed in New Mexico and Pennsylvania, and therefore any discussion of issues related to other states must considered within that context. In addition, my comments are not intended to create a legal representation but merely to respond to the limited facts presented by the question. Any opinion herein is not meant as a precise statement of legal rights or as a recommendation of any particular course of action. A more complete legal review can be obtained through local counsel.
If the provider's status changed as to their ability to prescribe, that would be a viable defense to a breach of contract suit. The details should be discussed with an attorney.
We do not have a client/attorney relationship until you make an appointment, we discuss your case face to face, I accept a retainer, and we explictly agree to enter into representation.