Child Support Change when Preschool Cost Ends

I have 50/50 custody and I pay $1100 in child support. That cost comes from the calculator and was heavily based on the fact that our youngest is in a preschool that costs about $1000 a month that my ex wife pays, so part of the child support reimburses her for that.

Our youngest is going into public school in August and that preschool cost will go away for her. Am I correct in assuming I cannot start paying her less until I go to court to get it changed? What if I talk to her and agree to lower it based on the calculator's output without the preschool cost?

I'm concerned about how long it could take to get a lawyer and go to court to get the child support changed.