title: Modify default value of field from mongo-engine django
body: <p>I have the following models  </p>
  
  <pre><code>Class xyz(Document):
      done_project_categories = ListField(StringField(), 
          default=['All', 'Residential', 'Corporate', 'Commercial'])
  </code></pre>
  
  <p>I want to change the default value as following and migrate onto mongodb</p>
  
  <pre><code>Class xyz(Document):
      done_project_categories = ListField(StringField(), 
          default=['All', 'Residential', 'Corporate', 'Commercial', 'Cafes'])
  </code></pre>
  
  <p>I am not able migrate onto datebase and looking for exact command to migrate the changes onto mongodb.</p>
  
  <p>I am using mongo-engine for to interact with the database from Django.</p>