title: Create a parent-child relationship after the parent type already exists
body: <p>I'm trying to add a new type as a child of an existing type. From <a href="https://www.elastic.co/guide/en/elasticsearch/guide/current/parent-child-mapping.html" rel="nofollow">ElasticSearch documentation</a>, it looks like this is possible but there is no details about how to do it. </p>
  
  <p>Here's what the documentation says:</p>
  
  <blockquote>
    <p>This must be done at index creation time, or with the update-mapping API before the child type has been created.</p>
  </blockquote>
  
  <p>There are no details about setting up a parent-child relationship in the <a href="https://www.elastic.co/guide/en/elasticsearch/reference/2.x/indices-put-mapping.html" rel="nofollow">update-mapping API documentation</a>. </p>
  
  <p>Intuitively, I think a parent-child relationship should be allowed to be established after a parent already exists since the parent does not need to know anything about the child.</p>
  
  <p>Here's the error I'm getting when I create a new type as a child:</p>
  
  <pre><code>{
  "error": {
    "root_cause": [
      {
        "type": "illegal_argument_exception",
        "reason": "can't add a _parent field that points to an already existing type"
      }
    ],
    "type": "illegal_argument_exception",
    "reason": "can't add a _parent field that points to an already existing type"
  },
  "status": 400
  }
  </code></pre>