(a) For good cause and at any time within the period allowed for appeal after the final termination of the administration of the estate of a decedent or ward, the court may vacate or modify an order or grant a rehearing. However, no such power shall exist as to any order from which an appeal has been taken or to set aside the probate of a will after the time allowed for contest thereof.
(b) No vacation or modification under this section shall affect any act previously done or any right previously acquired in reliance on such an order or judgment.