An example of how we might implement tiers that can be modified over time without losing the old information

tier_models.py

class Tier(models.Model):
    program = models.ForeignKey(Program)
    name = models.TextField()
    description = models.TextField()
    
    @property
    def current_version(self):
        return self.tier_versions.order_by('-active_at').first()
    
    @property
    def discount_amount(self):
        return self.current_version.discount_amount
    
    @property
    def income_threshold(self):
        return self.current_version.income_threshold

class TierVersion(models.Model):
    """
    An append-only data structure to update the financial details of a tier.
    Every time a tier is "updated", we actually create a new TierVersion
    object and associate it with the tier. This allows us to see the financial
    details of tiers as they were in the past.
    """
    tier = models.ForeignKey(Tier, related_name="tier_versions")
    discount_amount = models.IntegerField(null=False)
    income_threshold = models.IntegerField(null=False)
    active_at = models.DateTimeField(auto_now=True)

class FinancialAid(models.Model):
    user = models.ForeignKey(User)
    tier = models.ForeignKey(Tier)
    submitted_at = models.DateTimeField(auto_now=True)
    
    @property
    def tier_version(self):
        return (
            TierModification.objects
            .filter(tier=self.tier)
            .filter(active_at__lte=self.submitted_at)
            .order_by('active_at')
            .first()
        )
    
    @property
    def discount_amount(self):
        return self.tier_version.discount_amount
    
    @property
    def income_threshold(self):
        returns self.tier_version.income_threshold