The other way to change a custody agreement is through the courts. A parent can apply to amend the custody agreement, often seeking sole custody or primary custody. However, to amend the current plan, the applicant parent must prove that such an amendment is justified by a “substantial change in circumstances”. Any change in a co-parent`s household that could endanger or harm the children would be a good reason to seek a change. For example, a parent may learn that their ex-spouse abused drugs or alcohol (or relapsed after first seeking help for drug abuse). Another example may be that his co-parent has begun to become violent or abusive towards children. If a co-parent has undergone a major change in the place of work or working time, this could also justify a change in child custody.