755 ILCS 6/20-35
If a will has been admitted to probate under Section 20-20 before notice, any person entitled to notice under Section 6-10 of the Probate Act of 1975 may file a petition within 42 days after the effective date of the original order admitting the will to probate to require proof of the will, pursuant to this Section. The court shall set the matter for hearing upon such notice to interested persons as the court directs. At the hearing, the proponent shall establish the will by testimony of the relevant parties as provided in paragraph (1) of subsection (c) of Section 10-10, paragraph (1) of subsection (c) of Section 20-20, or paragraph (1) of subsection (e) of Section 20-20 or deposition of the relevant parties following the procedures in Section 6-5 of the Probate Act of 1975 or other evidence as provided in the Probate Act of 1975, but not as provided by paragraph (2) or (3) of subsection (c) of Section 10-10, paragraph (2) or (3) of subsection (c) of Section 20-20, or paragraph (2) or (3) of subsection (e) of Section 20-20, as if the will had not originally been admitted to probate. If the proponent establishes the will by sufficient competent evidence, the original order admitting it to probate and the original order appointing the representative shall be confirmed and effective as to all persons, including creditors, as of the dates of their entries, unless there is proof of fraud, forgery, compulsion, or other improper conduct that in the opinion of the court is sufficient to invalidate or destroy the will. The time for filing a petition to contest a will under Section 8-1 of the Probate Act of 1975 is not extended by the filing of the petition under this Section if the order admitting the will to probate is confirmed, but if that order is vacated, the time for filing the petition under Section 8-2 of the Probate Act of 1975 runs from the date of vacation of the order admitting the will to probate.
755 ILCS 6/20-35