Modifying custody

Divorced, 3 kids, joint legal custody, my ex has primary physical custody.

My oldest child (16 years old) is not doing very well at her mothers, and is failing school and doesn’t seem to care. My ex and I both decided that if she fails her grade, she should come to live with me, as where she is now just apparently isn’t working for her. If that happens, ideally I would like to modify child custody, just so my ex, in a fit of pique, can’t wave a court order in my face and yank my daughter back to her, regardless of what is best for the child. The problem is, I think if I approach her about legally modifying it in court, she would refuse.

If I have a notorized desire to modify child support, signed by her, stating the reasons (failing at school… discipline problems, etc), would this be any good to me at all, if she decides to pull my daughter back to her with the court ordered custody agreement? I understand that custody should be modified by the court, but I just don’t think she would agree to doing that, especially if I suggest it.

If custody provisions are contained in a court order, the only way to modify them is through another court order. A notarized agreement will not suffice.