title: Migrate GeoDjango model to change projection SRID
body: <p>I am attempting to use South to create a migration to convert my data from using the 4326 SRID to 900913.  After the migration, the coordinates remain in their 4326 format. (It's easy to tell the difference between the 4326 and 900913 projections, since the numbers are much larger in 900913)</p>
  
  <p>Here are the forward() and backward() functions from that migration:</p>
  
  <pre><code>class Migration(SchemaMigration):
  
      def forwards(self, orm):
          # Changing field 'ZipCoords.point'
          zips = orm.ZipCoords.objects.all()
          db.alter_column('itinerary_generator_zipcoords', 'point', self.gf('django.contrib.gis.db.models.fields.PointField')(srid=900913, null=True))
          for zip in zips:
              zip.point.transform(900913)
              zip.save()
  
      def backwards(self, orm):
          # Changing field 'ZipCoords.point'
          zips = orm.ZipCoords.objects.all()
          db.alter_column('itinerary_generator_zipcoords', 'point', self.gf('django.contrib.gis.db.models.fields.PointField')(null=True))
          for zip in zips:
              zip.point.transform(4326)
              zip.save()
  </code></pre>
  
  <p>I am checking their values using the Django Admin.  Also, interestingly, this migration "works" in reverse, it turns my coordinates into much smaller (but incorrect) numbers.</p>