755 ILCS 5/9-10
If it appears after entry of an order directing that original letters of office issue to an administrator that a person entitled to notice under subsection (a) or (b) of Section 9-5 or under Section 9-6 was omitted from the petition to issue letters or, if included in the petition, that notice to him was not mailed or published under subsection (a) or (b) of Section 9-5 or under Section 9-6, whichever is applicable, and that no waiver of notice was filed by the omitted or unnotified person, an amended petition shall be filed under the applicable Section or subsection which shall include the omitted or unnotified person. A copy of the amended petition and the order directing that original letters of office issue shall be mailed to or published for the omitted or unnotified person, as provided in Section 9-5 or Section 9-6, as the case may be, in the same manner as if the order were entered at the time the amended petition was filed. The original order directing that letters of office issue to the administrator is effective as to the omitted or unnotified person as of the date the amended petition is filed and is effective as to all other persons, including creditors, as of the date of entry of the original order.
755 ILCS 5/9-10