(a) Mental health treatment instructions contained in a declaration executed in accordance with this chapter supersede any contrary or conflicting instructions given by:
(1) a medical power of attorney under Subchapter D, Chapter 166, Health and Safety Code; or
(2) a guardian appointed under Title 3, Estates Code, after the execution of the declaration.
(b) Mental health treatment instructions contained in a declaration executed in accordance with this chapter shall be conclusive evidence of a declarant's preference in a medication hearing under Section 574.106, Health and Safety Code.