If a custody arrangement is in place and one of the parents wishes to modify it, s/he will need to prove that there has been a material change in circumstances. A material change in circumstances is something that alters the conditions of the child's life significantly enough that it may change the court's decision as to what is in the child's best interests.
A court decides custody for a child based on a careful weighing of many different factors. The court assesses, for example, the child's relationship with each parent, as well as the ability and desire of each parent to provide that child with the best home possible. After the court has made this assessment, the parties are bound by their decision and must live with it. The custody agreement it sets in place becomes legally binding and permanent in order to create stability for the child.
Because the court carefully considers their initial decision, they aren't just going to change the custody decision on a whim. Instead, the only way they will change it is if there is new information that could alter their original assessment of what is best for the child. This new information would be considered a material change in circumstances.
There are many different examples of a material change in circumstances. A few examples include:
The party seeking an alteration to the custody agreement based on the material change of circumstances will need to submit the appropriate papers to the court and gather the evidence necessary to prove that a change is being made, that the change is material, and that the change should result in a reassessment of custody. Hiring a lawyer can be helpful in successfully arguing these points.