Changing an existing family court order - which usually requires proving something has changed, and never happens by the parents' agreement alone.
Family court orders are meant to last, and the law makes them difficult but not impossible to change. The usual requirement is a substantial change in circumstances since the order was made, plus - for custody - that the change sought is in the child's best interests. The threshold exists to stop the same dispute being relitigated whenever one parent is dissatisfied, and it is applied seriously.
What qualifies differs by state and by what is being modified. For child support, a significant and lasting change in either parent's income, a change in the parenting-time split, or a change in the child's needs are the common grounds; many states also allow periodic review without proving anything, and some treat a deviation of a set proportion from the guideline figure as sufficient by itself. For custody, courts look for something meaningful and durable: a relocation, a change in a child's needs, a parent's circumstances materially altering, or a persistent failure to follow the existing order.
The most consequential point is procedural. An agreement between the parents does not change the order. Parents who informally agree that a child will now live with the other, or that a lower payment is acceptable, remain bound by what the court wrote: support continues to accrue at the ordered rate, arrears can build for years, and in most states they cannot be waived retroactively even by the parent owed them. The remedy is to have the agreement made into an order, which where both parents agree is usually straightforward.
Timing matters for the same reason. Modifications are generally effective no earlier than the date the request is filed, so the time between a change in circumstances and the filing is typically lost. A parent who loses a job and waits to see how things go is usually accruing an obligation calculated on income no longer earned.
The advice most worth having in this area is about timing, and it is usually free to act on: file when the circumstances change rather than when the situation becomes intolerable, because the gap is rarely recoverable. Advice is worth taking where income has dropped substantially, particularly for the self-employed, since what counts as income and whether a reduction was voluntary are both contested; where the parents have agreed to a change and want it made binding, which is the cheapest work a family lawyer does and prevents the most expensive problem in this entry; where a parent is relocating; where one parent has stopped following the order; and where the parents or the child now live in different states, because which court may modify is a threshold question and an order from a court without jurisdiction is worth little.
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