Nothing in this chapter shall be construed to modify or affect the terms of any declaration, appointment of agent or durable power of attorney validly executed prior to June 26, 1996, which grants the authority for medical treatment or directs the withholding or withdrawal of medical treatment, except that a prior declaration shall not be interpreted to allow the withdrawal or withholding of artificial nutrition or hydration unless that desire is specifically stated in that directive. If withdrawal or withholding of artificial nutrition or hydration is not specifically addressed in a prior declaration, a health-care provider shall comply with a decision regarding withdrawal or withholding of artificial nutrition or hydration for the patient made by a person then authorized to make health-care decisions for the patient to the extent the agent or surrogate is permitted by this chapter. Nothing in this chapter shall be construed to limit the use of any previous living will forms conforming to law or any other form which meets the requirements of this chapter.