Plan and document existing firewall
Planning and documenting the migration of the firewall is critical, without it you’re doomed. Over time configurations grow as rules and networks are added for specific purposes are often superseded or simply forgotten.
You need to document these as well as any other additional services or features (i.e. DHCP, DNS or VPN). This will assist you while you recreate the policies behind the firewall rules.
You can download a sample Firewall Planning Guide here: