Its is not possible to edit or change any values on the order that have affect on the order, meaning term, price, quantities or other values that affect the order.
There are though some non critical fields that can be edited even on active order press the Edit function. Its possible to edit some fields on:
1. Order information
2. Order product charge
Any changes that is done like this, not in Change order mode will only update the order meta data and not the order itself.