diff --git a/uncloud_pay/models.py b/uncloud_pay/models.py
index 1d95ae1..3f8498d 100644
--- a/uncloud_pay/models.py
+++ b/uncloud_pay/models.py
@@ -57,6 +57,10 @@ def end_before(a_date):
     """ Return suitable datetimefield for ending just before a_date """
     return a_date - datetime.timedelta(seconds=1)
 
+def start_after(a_date):
+    """ Return suitable datetimefield for starting just after a_date """
+    return a_date + datetime.timedelta(seconds=1)
+
 def default_payment_delay():
     return timezone.now() + BILL_PAYMENT_DELAY
 
@@ -333,6 +337,24 @@ class Order(models.Model):
 
         return self.starting_date + datetime.timedelta(seconds=self.recurring_period)
 
+    @property
+    def next_ending_date(self):
+        """
+        Return the next proper ending date after n times the
+        recurring_period, where n is an integer.
+        """
+
+        if self.recurring_period > 0:
+            now = timezone.now()
+            delta = now - self.starting_date
+
+            num_times = math.ceil(delta.total_seconds() / self.recurring_period)
+
+            next_date = self.starting_date + datetime.timedelta(seconds= num_times * self.recurring_period)
+        else:
+            next_date = self.starting_date
+
+        return next_date
 
     @property
     def count_billed(self):
@@ -698,7 +720,6 @@ class Product(UncloudModel):
 
 
     def create_or_update_recurring_order(self, when_to_start=None, recurring_period=None):
-
         if not self.recurring_price:
             return
 
@@ -709,6 +730,13 @@ class Product(UncloudModel):
             when_to_start = timezone.now()
 
         if self.last_recurring_order:
+            # If the new order is less in value than the previous
+            # order, the previous order needs to be finished first
+            if self.recurring_price < self.last_recurring_order.price:
+
+                if when_to_start < self.last_recurring_order.next_ending_date:
+                    when_to_start = start_after(self.last_recurring_order.next_ending_date)
+
             when_to_end = end_before(when_to_start)
 
             new_order = Order.objects.create(owner=self.owner,
diff --git a/uncloud_pay/tests.py b/uncloud_pay/tests.py
index 6586f3a..d9feed0 100644
--- a/uncloud_pay/tests.py
+++ b/uncloud_pay/tests.py
@@ -416,7 +416,7 @@ class ModifyProductTestCase(TestCase):
 
     def test_bill_for_modified_product(self):
         """
-        The bill should NOT contain a partial amount -- this is a BILL TEST :-)
+        Modify a product, see one pro rata entry
         """
 
         price = 5
@@ -429,16 +429,32 @@ class ModifyProductTestCase(TestCase):
         ending_date   = timezone.make_aware(datetime.datetime(2019,3,31))
         change_date   = timezone.make_aware(datetime.datetime(2019,4,17))
 
-        product.create_order(starting_date)
+        bill_ending_date = timezone.make_aware(datetime.datetime(2019,4,30))
 
-        bills = Bill.create_next_bills_for_user(self.user,
-                                                ending_date=ending_date)
+        product.create_order(starting_date)
 
         product.rc_price = 10
         product.save()
 
         product.create_or_update_recurring_order(when_to_start=change_date)
 
+        bills = Bill.create_next_bills_for_user(self.user,
+                                                ending_date=bill_ending_date)
+
+        # Sum:
+        # recurring1 = 5 CHF -> for 30days
+        # recurring2 = 5 CHF -> from 3rd of April to 3rd of May
+        # recurring3 = 10 CHF -> from 17th of April to ... 17th of May?
+
+        # If replacing lower order with higher:
+        # - close the lower order NOW
+        # - start higher order NOW+1s
+        # If replacing higher order with lower order:
+        # higher order continues until its end
+        # lower order starts after higher order+1s
+
+        self.assertEqual(bills[0].sum, price)
+
         # expeted result:
         # 1x 5 chf bill record
         # 1x 5 chf  bill record