gistfile1.py

class MyModel(models.Model):
    ...
    def save(self, *args, **kwargs):
        if self.pk:
            old_version = self.__class__.objects.get(pk=self.pk)
            # Now we have self and old_version, which are different
            # instances of the same object.
            if old_version.foo_field != self.foo_field:
                # foo_field is changed, do stuff.
        super(MyModel, self).save(*args, **kwargs)