CHILD SUPPORT

I fought a long hard battle to bring my son's deadbeat dad to justice of paying current child support & over $20,000.00 in arrearage.  The state of Missouri prosecuted him and I am supposed to get my first payment that is of both current and arrearage amout.  My son is 17 years old now.  He plans to go live with his dad in a couple of months.  My questions are, does my ex have to take legal action against me to modify the second court order thru the state if it is modifyable?  And will the monthly amount be changed since it also includes current along with arrearage.  My ex told my son that he called Child Support to notify them of the pending change of residency to get the amount changed.