Let me comment on the steps below and update where needed:
A) replace backup gateway
1) Establish SIC with the new Secondary-FW Gateway, change the version of the object and push policy with the "If installation fails do not install" option un-ticked.
2) Fetch the interface with Topology table on Secondary Firewall.
Never ever in a working cluster do this, all you need to do is update the interface names, when you change them, as both FW's use the same naming there should be no need if you move the cables to the same ports on the new gateway.
3) Check the CCP mode of existing Firewall and keep the same in Secondary New Firewall via CLI
This could help, after the last change of the new members change it to auto or unicast.
4) Check the Cluster-id of existing Firewall and New Firewall......If cluster unable to form then we have to change Cluster id of New Secondary Firewall.
Cluster will not form period. This is also what
@PhoneBoy said, different hardware will not work (unless they accidentally have the same number of cores.
5) Check the CoreXL status & fwaccel status on new secondary Firewall. If all the parameter are matched as per expectation we should see
"cphaprob stat" in READY state which means FW is ready for Failover
cphacu start - could be your lifesaver when you do get this right.
B) Failover by cpstop on old gateway.
C) Replace old gateway 1
D) Repeat step 1 and change the CCP setting
Regards, Maarten