A custody or support order is not necessarily permanent, but it is also not something either parent can simply ask a court to revisit whenever circumstances feel different. Most states require something specific first: a substantial change in circumstances since the existing order was entered.
Why this standard exists
Courts generally want custody and support arrangements to have some stability, both for the child's sake and so litigation does not become an ongoing, open-ended process. Requiring a substantial change in circumstances before reopening an order is meant to prevent relitigating the same facts the original order already accounted for, while still allowing for real, significant developments to be addressed (a parent relocating, a major change in income, a child's needs changing meaningfully, or a demonstrated safety concern).
What tends to count
While the exact legal standard and its application vary significantly by state and by judge, examples that commonly qualify as substantial changes include a parent's relocation that affects the existing schedule, a significant and involuntary change in either parent's income, a documented change in a child's medical, educational, or developmental needs, or evidence of a safety concern that did not exist or was not known at the time of the original order. Examples that commonly do not qualify include a parent simply disagreeing with the original terms, a minor scheduling inconvenience, or a change that was already anticipated when the original order was entered.
Why the standard is not the same everywhere
What counts as "substantial" or "material," what evidence a court expects to see, and what procedural steps are required to bring a modification request differ meaningfully from state to state. A description that is accurate for one state's courts may not describe what a different state's courts require.
Where this fits into a case
Whether a specific development meets the modification standard in a specific state, and what process is required to bring it before the court, is a question a licensed family law attorney in that state is positioned to answer.