Quick answer
An income change does not automatically reduce an existing order. Promptly ask about a modification.
Do now
Keep termination notices, benefit records, job-search records, and current financial information. Obtain the signed order and payment history. File the proper application if appropriate and keep proof of filing and service. Continue complying as far as possible while seeking advice about the balance due.
Local note
NJ has a modification packet; NY and DC have their own modification standards and procedures. Earlier unpaid amounts can remain owed even if a future payment changes. Voluntarily reducing income may be treated differently.
Official sources
NJ Courts — FD modification application packet, NY Courts — Starting a Case in Family Court, DC Courts — Parentage and Child Support Branch.