title: Django postgres make custom migration from CharField to JSONField
body: <p>I need to change this</p>
  
  <pre><code>data = models.CharField(max_length=500, null=True)
  </code></pre>
  
  <p>to this</p>
  
  <pre><code>data = JSONField(null=True, blank=True, default={})
  </code></pre>
  
  <p>From what I understand, I have to write custom migration. Closest info I managed to find is <a href="https://stackoverflow.com/questions/49274563/django-change-models-field-type-from-charfield-to-jsonfield?noredirect=1&amp;lq=1">here</a>, but I am completely clueless what to do with RunPython if that even correct thing to do here.</p>