@CMT_Chris if you connect it alongside the Sonicwall with both using the same IP then you will break your network. At the very best the Meraki firewall will just not work.
If most of your traffic is outbound then I'd say the easiest option is to temporarily connect the WAN port of the MX to the LAN of the Sonicwall and then start recreating the rules. You can then connect a computer to the LAN ports on the Meraki to test your copied rules. This won't risk service interruptions.
If you have internal services that external people connect to, like servers or a client VPN, then that is easy to swap over in a maintenance window.
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