title: Django: convert ManyToManyField to ForeignKey
body: <p>Let's supposed I created two models:</p>
  
  <pre><code>class Car(models.Model):
      name =  models.CharField(max_length=50)
      size =  models.IntegerField()
  
  
  class Manufacturer(models.Model):
      name =  models.CharField(max_length=50)
      country =  models.CharField(max_length=50)
      car = models.ManyToManyField(Car)
  </code></pre>
  
  <p>I added entries to both models, then I realized that each Car was only related to a unique Manufacturer. So, I should convert my ManyToManyField to a ForeignKey:</p>
  
  <pre><code>class Car(models.Model):
      name =  models.CharField(max_length=50)
      size =  models.IntegerField()
      manufacturer = models.ForeignKey(Manufacturer)
  
  class Manufacturer(models.Model):
      name =  models.CharField(max_length=50)
      country =  models.CharField(max_length=50)
  </code></pre>
  
  <p>How can I do that without losing my entries? I tried to look in South documentation but I did not found this way of conversion...</p>