How do I make a patch request to a document to change its type? This is resulting in an error Cannot modify immutable attribut

Cannot modify immutable attribute \\"_type\\""

{
  "mutations": [
    {
      "patch": {
        "id": "drafts.<document id>",
        "ifRevisionID": "<revision id>",
        "set": {
          "_type": "slidingBanner"
        },
        "unset": [
          "height"
        ]
      }
    }
  ]
}

S.documentTypeListItems().sort( /* sort by title */ )

export default () =>
  S.list()
    .title('Content')
    .items(
      S.documentTypeListItems().sort((a, b) =>
        a.spec.title.localeCompare(b.spec.title),
      ),
    );