title: apply field reshape to all documents in a mongodb collection
body: <p>I have had the following User schema with already a good amount of users saved in the db:</p>
  
  <pre><code>const userSchema = new Schema({
    ...
    country: String,
    authorities: [{ code: String, description: String }]
    ...
  });
  </code></pre>
  
  <p>I have decided to save authorities by country, so the schema becomes :</p>
  
  <pre><code>const userSchema = new Schema({
    ...
    country: String,
    authorities: [{
      country: String,
      authorities: [{ code: String, description: String }]
    }]
    ...
  });
  </code></pre>
  
  <p>My question is, how do I update already saved users to reflect schema update?
  NB: there is only one country right now.</p>