It´s possible for a buyer to send an order change. Both before and after an order response has been received from the supplier.
Order changes need to be responded to with a new order response.
If a buyer has sent an order change, the order status will be marked “Order changed” in the list of orders.
More information:
How to see in "Order details" which items have been affected by an order change?
What to take notice of when created an order response from an order with order changes?