title: Changing django default pk with AutoField to BigAutoField
body: <p>My model has a default pk with AutoField (integer) but later on i discovered that i need to use BigAutoField instead! 
  And also i have data in then with other models referencing the student model:: how do i change the pk field to BigAutoField and also reflects on other referencing models</p>
  
  <pre><code>class Student(models.Model):
      matric_no = models.CharField(max_length=20,  unique=True)  # first set it to U(random)
      password = models.CharField(max_length=128)
      session = models.ForeignKey(Session, null=True)
      programme = models.ForeignKey(Programme, null=True)
      school = models.ForeignKey(School, null=True)
      course_comb = models.ForeignKey(CourseComb, null=True)
      serial = models.IntegerField(null=True)
      current_level = models.CharField(max_length=3, choices=LEVEL_CHOICES, default='100', null=True)
      last_login = models.DateField(null=True)
      is_active = models.CharField(max_length=1, default=1, choices=((1, 1), (0, 0)))
      created_at = models.DateTimeField(default=timezone.now)
      updated_at = models.DateTimeField(auto_now=True)
  </code></pre>
  
  <p>a model referencing Student</p>
  
  <pre><code>class Profile(models.Model):
  
      student = models.OneToOneField(Student, on_delete=models.CASCADE)
      attachment = models.ImageField(null=True, blank=True, verbose_name="Profile Image")
      surname = models.CharField(max_length=255, null=True, verbose_name="Surname")
      othernames = models.CharField(max_length=255, null=True, verbose_name="Othernames")
      SEX_CHOICES = (
        ("M", "Male"),
        ("F", "Female")
      )
  </code></pre>