In Massachusetts, the standard to modify an existing child custody order requires showing a “material and substantial change in circumstances” since the original order was issued, and that the requested modification is “in the best interests of the child”. The change must be significant and directly affect the child’s welfare, not just a minor preference or inconvenience. Examples of valid changes include a parent’s relocation, substance abuse, neglect, or significant changes in work schedules or living conditions. 

If both parents agree on a Modification, they can file a Joint Petition, which the court still needs to approve, but if not, there are requirements more than the parents want to change; things have to be a significant change in circumstances that you can demonstrate.