In a Master Detail Relationship, records cannot be reparented. However, to allow child records in master-detail relationships on custom objects to be reparented, enable “Allow reparenting” option in the master-detail relationship definition.

When this option is enabled, users can change the parent of the child records. When the parent is changed for a child record, then the child record access will move to the new parent record since sharing for the child record is inherited from the parent record in a master detail relationship.