From Frequently Asked Questions – CMS.gov/Jimmo:
Q11: If a patient is not improving or is not expected to return to his or her prior level of function from skilled nursing or therapy, does Medicare coverage for skilled nursing or skilled therapy services stop unless the patient deteriorates?
A11: The Medicare program does not require a patient to decline before covering medically necessary skilled nursing or skilled therapy. For a patient who had been expected to improve, but is no longer improving, a determination as to whether skilled care is needed to maintain the patient’s current condition or prevent or slow further deterioration must be made, and if such skilled care is needed, a plan of care to reflect the new maintenance goals must be developed. … [Emphasis added.]