First of all child support is the childs not the parents. So, a parent can not make agreements on child support (that are enforceable). So, her getting child support back takes but a phone call.
Courts like to maintain status quo. That is the barrier to changing placement of the child. But you can always ask. At least until AB 29 is passed. AB 29 limits your ability to make such requests to the court. It increases the requirements for the first 2 years and makes what we have now in the first 2 years for the rest of the childs life. Basically the statute will reduce changes for the balance once passed.
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