Another option you could do is create a new network and clone the configurations from the existing network.
Go to Organization -> Inventory. Select your new appliance. Select Add To... and the choose New Network and copy the configuration from your network you want to replace. Except for Site-to-Site VPN settings and some appliance Name settings, every other setting will be exactly the same. Make sure the Internet port isn't using the same IP address until you are ready to migrate. Plug it in and have it access the internet like
@Nash said, then when you are ready perform the swap.
I have done this when I we needed to upgrade our MX100 Firewall's with MX250s and wanted minimal down time. Keep in mind if you are doing this with appliances that use Public Static IPs, you may have you contact your ISP provide and have them clear the arp cache on that IP address.
CMNO, CCNA R+S