Yes, you can update the tracking number and carrier/class on existing
shipments. Updating an order that is already shipped will result in a
new email being sent to the buyer.
If you submit your update as a partial shipment notification, we will
identify which shipment to update by matching the SKUs and quantity in
the notification with the SKUs and quantity in the existing
shipments. If we don't find a match or if you submit your update as a
full shipment, we will update the least-recently updated shipment
instead.