title: Change type of Django model field from CharField to ForeignKey
body: <p>I need to change the type of a field in one of my Django models from <code>CharField</code> to <code>ForeignKey</code>. The fields are already populated with data, so I was wondering what is the best or right way to do this. Can I just update the field type and migrate, or are there any possible 'gotchas' to be aware of? <strong>N.B</strong>: I just use vanilla Django management operations (<code>makemigrations</code> and <code>migrate</code>), not South.</p>