Mich. Comp. Laws § 556.503

Sec. 403. Except as otherwise provided in this act, on the effective date of this act all of the following apply: (a) Except as provided in subdivision (c), this act applies to a power of attorney created before, on, or after the effective date of this act. (b) This act applies to a judicial proceeding concerning a power of attorney commenced on or after the effective date of this act. (c) This act applies to a judicial proceeding concerning a power of attorney commenced before the effective date of this act unless the court finds that application of a provision of this act would substantially interfere with the effective conduct of the judicial proceeding or prejudice the rights of a party, in which case that provision does not apply and the superseded law applies. (d) An act done before the effective date of this act is not affected by this act.