Threatening court over 6 year old oral agreement

I live in Fresno, CA. My ex and I got divorced and have never been back to court since. We’ve made many changes to our original support agreement on our own. One of those changes is coming back to bite me. My original support amount was $1450 a month. 6 years ago we came to an agreement to change it to $1100 a month. Since then for the past 6 years I’ve paid $1100 a month. It’s been fine up until now, as the kids are reaching adulthood. One of the kids turned 19 recently and the other is 17. As they’re starting to drop off child support my ex now has a desire to file a claim and go back and collect the difference between the $1450 and $1100 for the past six years. I have all my receipts for the $1100 payments for the last 6 years. Is there any recourse against my ex claiming back due child support in this situation?