title: Change IntegerProperty to FloatProperty of existing AppEngine DataStore
body: <p>I built an appengine application (python) which need to convert existing datastore entities in integer value (100) to float value (100.00) for currency conversion issue.
  How's the right way doing this? Since my query returning error when i just change property type in my model.</p>
  
  <p><strong>Old Model:</strong></p>
  
  <pre><code>class Learn(search.SearchableModel):
      pid = db.ReferenceProperty(Product, collection_name='picks')
      title = db.StringProperty()
      description = db.TextProperty()
      order = db.IntegerProperty()
      cost = db.IntegerProperty(default=0)
      cost1 = db.IntegerProperty(default=0)
  </code></pre>
  
  <p><strong>New Model:</strong></p>
  
  <pre><code>class Learn(search.SearchableModel):
      pid = db.ReferenceProperty(Product, collection_name='picks')
      title = db.StringProperty()
      description = db.TextProperty()
      order = db.IntegerProperty()
      cost = db.FloatProperty(default=0.000)
      cost1 = db.FloatProperty(default=0.000)
  </code></pre>
  
  <p>I need a proper way to alter this datastore property type without changing (delete old and add new) existing data. Since it's key used in many others table/model.   </p>
  
  <p>Thanks.</p>