Mont. Code § 72-3-1002
If one or more heirs or devisees were omitted as parties in or were not given notice of a previous formal testacy proceeding, the court, on proper petition for an order of complete settlement of the estate under 72-3-1001 and after notice to the omitted or unnotified persons and other interested parties determined to be interested on the assumption that the previous order concerning testacy is conclusive as to those given notice of the earlier proceeding, may determine testacy as it affects the omitted persons and confirm or alter the previous order of testacy as it affects all interested persons as appropriate in the light of the new proofs. In the absence of objection by an omitted or unnotified person, evidence received in the original testacy proceeding shall constitute prima facie proof of due execution of any will previously admitted to probate or of the fact that the decedent left no valid will if the prior proceedings determined this fact.
§ 72-3-1002, MCA