Change order status string from "Completed" to "Shipped" I'd like to change the order status string from "Completed" to "Shipped" to allow for better communication between us and the customer.

Commerce Custom Order Status would sidestep this problem by allowing me to create a new status, but that isn't the best way around it, because there might be triggers set for "completed".
The best thing for you to do would be to alter the order status info array to give the "Completed" status a new title. The example in the documentation actually demonstrates specifically how to change the title of the completed status. Look for hook_commerce_order_status_info_alter() in commerce_order.api.php.

There are ways to override strings through the user interface, but this is such a small module change that I'd really recommend directly making it in a site-specific module.