N either support nor custody is ever etched in stone. These orders are always modifiable through the court.
Child support is always modifiable if one's income or job status changes.
Once an order is entered, you MUST pay your support through the court system. If you don't pay support pursuant to an order, you can be put in jail. There are many other items that are paid in addition to just support, day care, medical bills, etc. If there is a change in circumstances and you believe the order should be modified, then file a petition. Do not just unilaterally change what you are paying.