diff --git a/doc/README-billing.org b/doc/README-billing.org
index d749e31..83c133b 100644
--- a/doc/README-billing.org
+++ b/doc/README-billing.org
@@ -3,10 +3,31 @@
    - Orders are the heart of uncloud billing
    - Have a starting date
    - Have an ending date
+   - Orders are immutable
+   - Can usually not be cancelled / cancellation is not a refund
+     - Customer/user commits on a certain period -> gets discount
+       based on it
+   - Can be upgraded
+     - Create a new order
+     - We link the new order to the old order and say this one
+       replaces it
+     - If the price of the new order is HIGHER than the OLD order,
+       then we charge the difference until the end of the order period
+     - In the next billing run we set the OLD order to not to bill anymore
+     - And only the NEW order will be billed afterwards
+   - Can be downgraded in the next period (but not for this period)
+     - We create a new order, same as for upgrade
+     - The new order starts directly after the OLD order
+     - As the amount is LOWER than the OLD order, no additional charge is done
+       during this order period
+   - We might need to have an activate datetime
+     - When to implement this
+   - Order periods can be
 *** Statuses
     - CREATING/PREPARING
     - INACTIVE (?)
-    - TO_BILL / NOT_TO_BILL
+    - TO_BILL
+    - NOT_TO_BILL: we use this to accelerate queries to the DB
 *** Updating status of orders
     - If has succeeding order and billing date is last month -> set inactive
 ** Bills
@@ -14,7 +35,6 @@
    - Can be preliminary
 *** Which orders to include
     - Not the cancelled ones / not active ones
-    -
 ** Flows / Approach
 *** Finding all orders for a bill
     - Get all orders, state != NOT_TO_BILL; for each order do:
@@ -25,6 +45,38 @@
             - get_or_create_bill_for_this_month
             - assign bill to this order
             - set to NOT_TO_BILL
-
+      - is it a recurring order?
+        - if it has a REPLACING order:
+          -
     - First of month
     - Last of month
+*** Handling replacement of orders
+    - The OLD order will appear in the month that it was cancelled on
+      the bill
+    - The OLD order needs to be set to NOT_TO_BILL after it was billed
+      the last time
+    - The NEW order will be added pro rata if the amount is higher in
+      the same month
+    - The NEW order will be used next month
+**** Disabling the old order
+     - On billing run
+     - If order.replacement_order (naming!) is set
+       - if the order.replacement_order starts during THIS_MONTH
+         - add order to bill
+       - if NOT:
+         - the order was already replaced in a previous billing period
+         - set the order to NOT_TO_BILL
+**** Billing the new order
+     - If order.previous_order
+*** Handling multiple times a recurring order
+    - For each recurring order check the order.period
+      - Find out when it was billed last
+        - lookup latest bill
+      - Calculate how many times it has been used until 2359, last day
+        of month
+        - For preliminary bill: until datetime.now()
+        - Call the bill_end_datetime
+        - Getting duration: bill_end_datetime - order.last_billed
+          - Amount in seconds; duration_in_seconds
+        - Divide duration_in_seconds by order.period; amount_used:
+          - If >= 1: add amount_used * order.recurring_amount to bill