If you have a combined network temporarily split it up. Then make a new network which is a clone of the resulted appliance network and add the new MX devices to that. In this stage all the config will already be copied with the exception that if you have a site to site autovpn, it will be disabled for this network since you would have duplicate networks. Then once the new HA pair is online, disable the old MX'es, cable up the new ones. Once online enable your site 2 site vpn and you are ready. Do mind that your dynamic DNS name will be different. So if you are using it for port forwarding or client VPN make sure the other sides are ready to change their config or if you use a DNS CNAME, adjust it. Once online you can now proceed and merge the networks with the new appliance network and then you are ready.
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