Explanation/Reference:
To minimize the effort required to restore the configuration on a managed firewall involving a Return Merchandise Authorization (RMA), replace the serial number of the old firewall with that of the new/ replacement firewall on Panorama. To then restore the configuration on the replacement firewall, either import a firewall state that you previously generated and exported from the firewall or use Panorama to generate a partial device state for managed firewalls running PAN-OS 5.0 and later versions. By replacing the serial number and importing the device state, you can resume using Panorama to manage the firewall.
References: https://www.paloaltonetworks.com/documentation/61/panorama/panorama_adminguide/ troubleshooting/replace-an-rma-firewall