I'll tell you how we had to do it from our point of view... (NCP)
My SO is the one that requested the modification. He had to do that in writing. He wrote a letter to CSE, and once they got it, they sent him HIS paperwork to fill out. They also sent the CP HER paperwork to fill out. Each one had to attach either last years tax returns, copies of W-2's, or 2 recent paycheck stubs when they sent the paperwork back in. The only "documents" my SO had to send in were his checkstubs, W-2, or tax return. We sent in his check stubs AND his W-2's as added insurance. Of course, he had to fill out the paperwork they sent him as well that wanted to know about bank accounts, property owned, vehicles owned, etc. but since he has none of that he didn't have to send that documentation in. His ex had to also fill out the same paperwork and send in HER check stubs, W-2's or income tax return as well.
Then, about 2 weeks later we got a response from CSE that said his CS should be reduced based on their incomes, blah, blah, blah. In that was the Motion For Modification of Child Support. He had to reply to them within 30 days to let them know if he agrees with their figures or not. He did, and he did. His ex also got the same thing... the cover letter, the calculated Form 14 for Missouri CS, and the Motion. She also had 30 days to respond and let them know if she agreed. She did, and she did. They both called the person handling the mod. that day and let them know it was fine, to go ahead with it.
This was on February 24th. It's still not done yet. That's why I said that it'll be "hurry up and wait" time.
