How much you pay in Wisconsin can be changed but must be by court order since the original amount was court ordered.
http://www.dwd.state.wi.us/dwd/publications/dws/child_support/dwsc_864_p_1.htm#Changing_Your_Support_Order
Changing Your Support Order
I cannot afford to pay the amount of support that the court has ordered. What can I do?The child support agency may do a review of your court order if you can show that there has been a substantial change in circumstances. (For example, if your place of employment closes, or if you now have equal placement of your child. Any change in placement has to be approved by the court in order to be a substantial change in circumstance.)
If it has been more than three years since your order was set by the court or last reviewed, you may ask your child support agency to review your court order. (Every three years, the Bureau of Child Support will send a notice to you and the other parent to remind you of your right to ask for a review of your child support order.)
If the child support agency finds that the order is too high or too low, the agency will ask the court to change the order. The agency may write up a stipulation (legal agreement) for an order change and ask both parents to sign it. An agreement made by the parents must also receive court approval to be valid.
It is important to tell your child support agency about substantial changes. Only a court can change the order, but your child support agency may be able to help you.
When reviewing a support order, the agency and court will take into account whether or not the change in your earnings is due to a change in your “ability to earn.” For instance, your ability to earn might change if you are laid off from your job. However, if you decide to reduce your hours to part-time, your ability to earn has not changed. (Click here for more information about “ability to earn.”)
If you believe your order should be changed, but the child support agency does not agree, contact your family court for information about how to ask the court to review your child support order. Some courts have a "do it yourself" (pro se) package available that you may use to ask the court for a review. You and the other parent may reach an agreement on your own. The court or family court commissioner can provide the state court form for the agreed changes in family court cases. You may also hire a private attorney to help you. Click here for court forms.