title: Magento: Can I change the discount on an order that is payed for?
body: <p>We are developing this Magento site where we need to be able to add discounts on a order after the order is payed for (order has a invoice). Is this possible?</p>
  
  <p>All is done in the backend.</p>
  
  <p>This is our scenario:
  1. Backend user adds order.
  2. Order is payed for.
  3. Invoice is created.
  4. We programmatically add a new product to the order.
  5. Magento creates a new invoice for the new product, with the extra costs.</p>
  
  <p>On step 4 we would also like to re-calculate the allready given discounts, or replace the existing discount with a new one. Because we also work with percentage discounts it needs to be recalculated on the order.</p>