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Yes, I think this new evidence would fall under the substantial change in circumstances test. Furthermore, demonstrating that a fraud was perpetrated on the court could result in the judge even awarding you more custody and child support. I think you should certainly file for the modification and if the other parent had an attorney, file for sanctions against that attorney for this.
Actually, I'm glad you told me that, because I used to work in the Texas Child Support government agency and overpayments even by error were generally considered gifts and you do not usually get credit. I do still think you should file a motion to modify or motion for sanctions against them for the fraudulent conduct.
The motion for sanctions would be against the attorney if he had one, but not against him. For him, you would be asking the judge to set aside the order and set a proper amount and release any arrears. I think you should raise the issue now, though you could not necessarily bring it up as a legal issue again, you should show that this is the type of conduct he engages in if you were to seek in custody.
You might be able to get sanctions (which is really just like a fine) against that office's attorney if they had one.
How dis you get this information? Would you like to do a phone call for an additional charge? It would allow you to explain the entire situation so I can provide the correct information to you.