You are safer hiring experienced help to prepare your response, and to spot future problems, that court-provided documents often don't reveal. Once the final Decree or Judgment has been signed, it may be too late. In this state the factors under the statute for the court's concern are primarily the Health, Safety and Welfare of the child. In addition to the physical matters including hygiene, medicine, nutrition, and supervision the court will look at emotional and psychological factors. In particular, domestic violence in the home is considered to be detrimental to children.
Public policy is to assure that children have frequent and continuing access to both parents.
Both parents should ideally cooperate with each other and share the rights and responsibilities of parenting in order to provide for the best interests of the child. A skillful attorney will argue that you've acknowledged the appropriateness of the other parent as a caretaker by your own action in leaving the child in their care. This can be effectively used to argue maintenance of the status quo. The Burden of Proof issue becomes important when it means the difference in how you must prove that a substantial reason exists to change the existing custody arrangements. It will be harder to prove if you need to show it by clear and convincing evidence.