Mich. Comp. Laws § 700.2503: Writings intended as wills.

Sec. 2503.

Although a document or writing added upon a document was not executed in compliance with section 2502, the document or writing is treated as if it had been executed in compliance with that section if the proponent of the document or writing establishes by clear and convincing evidence that the decedent intended the document or writing to constitute any of the following:

The decedent's will.

A partial or complete revocation of the decedent's will.

An addition to or an alteration of the decedent's will.

A partial or complete revival of the decedent's formerly revoked will or of a formerly revoked portion of the decedent's will.