Примерно так:
class MySuperModel(models.Model):
my_super_field = models.CharField("My super field", max_length=1024)
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
self.previous_my_super_value = self.my_super_field
def save(self, *args, **kwargs):
super().save(*args, **kwargs)
before = self.previous_my_super_value
after = self.my_super_field
