JED 1.6.0+ - Changing a dimension ID

{
    "dimensions":
    [
        {
            "dim": 123,
            "unregister": true
        },
        {
            "dim": 456,
            "dimensiontype": {
                "existing_dimensiontype": "dimension_name_here"
            }
        },
        {
            "dim": 789,
            "dimensiontype": {
                "id": 789,
                "name": "Some dimension",
                "worldprovider": "world.provider.name.here.SomeWorldProvider",
                "require_exact_match": true
            }
        }
    ]
}