Just because a divorce has been finalized, irrespective of whether it was through a court decree or a mutually agreed upon settlement agreement, it does not mean it is set in stone and forever exempt from being changed or amended in any way.
The fact of the matter is that either spouse could still challenge some court decisions by filing an appeal. Either spouse, or both simultaneously, have the right to appeal a judge's decision to a higher appellate court. While court decisions are subject to appeal, a settlement agreement is typically not if both parties mutually agreed to the settlement's terms. So, for example, if both parties mutually reached a settlement agreement that laid out the terms on how much the child support payments will be and who will take ownership of what estate, and that settlement was approved by a judge, then chances are the spouses will have to live with those decisions.
However, parties have the option to file motions for modification of certain provisions in a divorce decree such as the child support payments and child custody arrangements. This legal mechanism is available to parties when there is substantial change in one's circumstances or in the needs of the minor child that require a fresh review.
For example, if a spouse is awarded custody and the other is ordered to pay child support but subsequently lose their sole source of income, the spouse paying child support can file a motion to modify the agreement in light of them being laid off to temporarily lower their payment until they land a new job or get some other source of income.
Source: FindLaw, "Appeals and Motions to Modify the Divorce Judgment," Accessed Aug. 24, 2015