Yes. Assuming the item/order is not already in production you can change the order details. The usual need for this is to change the shipping address.
You cannot request partial fulfillment of items in an order, this will result in a failure. If you need partial fulfillment, you will need to create a new order.
With print-on-demand, all items are custom printed and never pre-produced.
Please note that once the was the order is in production, there is no option to change the order details. The order is in production once the badge is shown on the order detail page. Please see below for an example of an order in production.