They will say no and it was your job to inform them sooner that it should have been ended. The system doesn't keep tabs. They just pull what they are told to until they are told to stop. They don't check that the same conditions exist before each payment is withheld / made. 

Ahhhhh. The great socialist republic of SpermLand. (OR).  I know that court and CS idiocy well. Our CO was issued in OR and the toothless moronic crap around navigating family courts there is nuckin futz. 

Overt bias to idiots who have no business with access to any kid, idiots who work for the courts and CSE offices, excuse making entitlement maximizing low level govt beurocrats, etc....

We had to learn the system better than those who work for the system know the system so that we could protect our son (my Skid) from the toothless morons in the SpermClan. 

We finally learned that we could initiate change of venue to Texas once SS had resided in Texas for 6mos.  Judges in Texas can't stand their counterparts in OR and were itching to nail the SpermIdiot for a ton of CS. So we kept that as a big stick to smack the SpermClan with when they got out of line.  We pretty much played the OR system or the TX option whichever was to our advantage.  We would make sure to provide an updated TX CS chart so they clearly understood what they would be paying if they pissed  us off.

My SS could have kept his SpermIdiot's balls nailed to the CS wall for four more years after he turned 18 and graduated from HS as long as he was a full time college student if he had wanted to. But he let the toothless dipshits guilt him into letting them off of the hook.  When we discussed this with SS he just said... The money doesn't matter to me and they will just bitch and cry if I force them to keep paying CS while I am in college.  He was 18.  There wasn't anything we could do about it. Unfortunately.

They notified the CSE office on  his 18th birthday to stop CS.  It would have been a complete PITA to reinstate CS if SS had chosen to.  While the SpermClan didn't shit about the CO they sure knew how to end CS ASAP when SS aged out from under the CS order.

Good luck in recouping the over payment. It isn't likely that you can make that happen.  OR regulations are clear that either party can submit for a CS change when certain milestones are met.  In our CO the SpermIdiot paid half of SS's after school care until age 12. At age 12 either they or we were supposed to call the CSE office to end the after school care element of the CS amount.  We sure didn't do it... and neither did they.  So ... they kept paying that part of the CS for 6 more years. It wasn't much but we were not going to those toxic diphsitiots any favors.

bear in mind that local CSEUs get FEDERAL KICK BACKS based on the dollar amount of CS they collected so there's NO WAY they will allow you to recoup the amount on which they've already received their kickback administrative fees.

The system certainly is broken

I live in Oregon and we just got done with custody and the mess of CS. Tax returns? Gone! Even though one of the children DH claimed is our own mutual child and not BM's but she was still getting his tax refunds to pay his arrears from when the economy crashed. Then we got active and started contacting the Case Manager. They say you can contact CS by email. They forward it to your Case Manager and from there you can continue the correspondence of any questions you have. I also searched through their forms and found the Modification of Child Support and Credit Towards Arrears for the time we have had SD. I would file for modification because the substantial change is that she isn't going to school. They should freeze the account until it can be decided. School transcripts should be able to provide them the information but it usually helps to do the leg work yourself and have it ready for them. With our situation CS paid out to us our next tax returns. A few times they sent DH's employer more papers for garnishment but with an email to the caseworker we got it figured out. So basically utilize the communication with the caseworker. It's also important to state that the current situation puts a large financial burden on your family. Just don't get discouraged from any setbacks you experience with CS. Keep communicating with them, fill out their forms, and get the Child Attending School discovered as a child not attending school. From my experience they reply to emails in the next 1 to 2 business days. I think emails are better than phone calls because you can say everything you need to say the way you want to say it and you can add attachments to back you up.