title: Django migration error involving m2m field
body: <p>I am trying to modify my creator field from a ForeignKey to a ManyToManyField in hope of being able to select many users to be the creator.</p>
  
  <p>When I make the change in my code, I receive the following error when making the migration:
  "You cannot alter to or from M2M fields, or add or remove through= on M2M fields."</p>
  
  <p>Would anyone know how to make this change possible for the migration to be successful?</p>
  
  <p>Please see my code below.</p>
  
  <p>Thanks in advance!</p>
  
  <pre><code>class Event(models.Model):
      title = models.CharField(_("Name of client"), max_length=100)
      creator = models.ForeignKey(settings.AUTH_USER_MODEL, null=False, blank=False, verbose_name=_("client"),
                              related_name='creator')
      start = models.DateTimeField(_("start"))
      end = models.DateTimeField(_("end"), help_text=_("The end time must be later than the start time."))
      calendar = models.ForeignKey(Calendar, null=True, blank=True, verbose_name=_("calendar"))
  </code></pre>