title: MongoDB Replace _id field with existing field Id
body: <p>I have an existing mongo collection. Each object inside looks similar to this:</p>
  
  <pre><code>{
    "_id": ObjectId("..."),
    "Id": 4321,
    ...
  }
  </code></pre>
  
  <p>The <code>"Id"</code> field I have is unique to each object.</p>
  
  <p>Is it possible to run a <code>db.collection.update</code> query in such a way as I can replace the "_id" field with the contents of "Id"?</p>
  
  <p>i.e.</p>
  
  <pre><code>{
    "_id": 4321,
    ...
  }
  </code></pre>