Custody, visitation, and child support may be changed after a divorce decree is finalized.  Either parent may file a motion to modify with the court that last entered an order regarding the children.  If the children have lived in a different country for six months, a motion to transfer the case to that county may be also be filed by the parent filing the modification motion or the parent responding to it.
Custody and visitation can also be modified with the court changing who has primary possession and/or the visitation schedule if the circumstances of the child or one or both parents have materially and substantially changed or the current order has become unworkable or inappropriate.  Failure to give notice of address changes or of inability to pick up the children may also justify changes.
An order of child support may be modified at any time upon a showing of substantial change in circumstances.  Generally, an order of child support may be modified one year or more after it has been entered without showing a substantial change in circumstances if: the order works a severe economic hardship on either party or the child is no longer in the age category on which the current support amount was based; and the child is still in high school and there is a finding that there is a need to extend support beyond the eighteenth birthday to complete high school.
The first step to a modification for child support is the filing of a petition to modify.  Then you need to swerve the other party with a summons, and a copy of the petition.  Once this is done, you need to file proof of service with the court.  In order to serve the other party correctly, it is best to hire a process server.  The responding party’s answer must be returned to you (or your attorney) and the answer filed with the court within twenty days after service of the petition.