When you switch to a new BlackBerry device, during setup, you will be prompted to enter your BlackBerry ID username and password. It's important to use the same BlackBerry ID that you used on your previous device, since many of your apps and settings are associated with your BlackBerry ID.
For instance, when you use your existing BlackBerry ID on your new device, you can reinstall apps that you downloaded on your previous device from the BlackBerry World storefront, if a version of that app exists for your new device.