There isn't any easy way. Just lessons learned. Screenshot everything on the Sonicwall. Backup config and firmware. Setup MX as best as possible before WAN switch over. Setup S2S VPN if needed. Setup VPN profile on clients if needed. Then cut over. Leave the Sonicwall up perferrably in a DMZ or Vlan or not even physically connected to the same network. That way any gotchas you can get in and check the exact settings and config.