404.847.  Prior durable power of attorney remains valid, when. — Nothing contained in sections 404.800 to 404.865 shall be construed to invalidate any durable power of attorney executed prior to August 28, 1991, which permits an attorney in fact to make health care decisions for the principal.  The provisions of sections 404.710 and 404.820 henceforth apply to durable powers of attorney for health care executed prior to August 28, 1991.  In the absence of a specific writing, decisions regarding nutrition and hydration must be made in accordance with state and federal law.