Michigan Compiled Laws, Chapter 700. Estates and Protected Individuals Code § 700.3406

Sec. 3406. (1) If evidence concerning execution of an attested will that is not self-proved is necessary in a contested case, the testimony of at least 1 of the attesting witnesses, if within the state and if competent and able to testify, is required. Due execution of an attested or unattested will may be proved by other evidence.

(2) If a will is self-proved, compliance with signature requirements for execution is conclusively presumed and other requirements of execution are presumed subject to rebuttal without the testimony of any witness upon filing the will and the acknowledgment and sworn statements annexed or attached to the will, unless there is proof of fraud or forgery affecting the acknowledgment or a sworn statement.

(3) If a witness is competent at the time he or she signs the will, the witness's subsequent incompetency from whatever cause does not prevent admission of the will to probate, if it is otherwise satisfactorily proved.