The custody decision in a divorce decree is not final. It can be revisited when circumstances change.
Grounds
Abuse, severe neglect, addiction or harmful conduct, forced religious change of the child, unauthorised relocation abroad, the child's preference (at sufficient age).
Evidence
Specialist reports, school records, medical reports, photos and videos, family witnesses. Objective evidence is decisive.
Court
The family court at the child's residence. Both parents and the child (depending on age) usually appear in person.
Timing
Six months to a year. In urgent cases (real risk to the child), a temporary order can be obtained in weeks.