The Bradley Amendment to the US Civil Code( 42 U.S.C. § 666(a)(9)(c) ) requires state courts to prohibit retroactive reduction of child support obligations. If your ex was to request a CS modification based on changed economic circumstances and the request was granted , any lesser obligation would only be retroactive from the date the petition was filed .
-------------------- Never let your sense of morals get in the way of doing what's right. Isaac Asimov