Hey @MtxMan ,
As promised in my last reply, Im sending you steps TAC gave me almost 2 years ago when customer needed this done. Since it does not let me attach a file here, I pasted the actual link he sent us a reference (Version is R80.30, as thats what customer was on back then, but Im positive process is exactly the same for any version)
https://sc1.checkpoint.com/documents/R80.30/WebAdminGuides/EN/CP_R80.30_ClusterXL_AdminGuide/html_fr...
https://dl3.checkpoint.com/paid/48/4808360334cfd91e38eb192da36ea686/CP_R80.30_ClusterXL_AdminGuide.p...  (same doc, just in pdf format)
Below is exactly what TAC guy sent us and we followed it and worked fine. Its pretty much boils what @PhoneBoy described in layman terms.
Andy
Process:
The documentation mentions the Standalone deployment for those who have a Standalone firewall and would like to convert it to ClusterXL. In your situation, you can go straight to page 151. "Creating the ClusterXL Object"
Computer B refers to your new firewall and Computer A is your current firewall. 
Basically here are the steps: 
- Install and configure the new cluster member. (Computer B)
- make sure that the new firewall can talk to the old firewall and vice versa. 
- Configure the local configuration such as authentication server, hostname, static route, dynamic route etc.
- In the policy, remove any references to the old firewall.
- Create a new cluster object in SmartConsole.
- Configure the interfaces, Antispoofing, Office mode etc. 
- The cluster VIP will be the old firewall local IP
- Open the Cluster object and in the "Cluster Members" page, click Add, and select New Cluster Member.
- Establish SIC
- Get interface without topology 
- Define a Sync interface 
- Install the policy on the cluster currently including member B only. 
- On the old firewall. 
- Disconnect all proposed cluster and Synchronization interfaces. New connections now open
 through the cluster, instead of through computer 'A'.
- Change the addresses of these interfaces to some other unique IP address which is on the
 same subnet as computer B.
- Connect each pair of interfaces of the same subnet using a dedicated network. Any hosts or
 Security Gateways previously connected to the Security Gateway must now be connected to
 both members, using a hub/switch.
- Update the topology of the Security Gateway that you just added by clicking Get Topology without interface.
- In the Cluster Members page, click Add and select "Add Security Gateway to Cluster" 
- Select the old firewall 
- In the "Edit Topology" page, determine the interface type. 
- Configure the Policy base. (VPN domain, rule base, NAT if needed)
- Install the policy. 
	Best,
Andy