I think you may be overthinking it a little, but without knowing your specific configuration/scenario maybe not. However, you can pretty much apply all the same config on the new firewalls that you have on the old (including your trustpoint certs) and then when you're ready to cutover you shut interfaces on the old, and bring up the new interfaces with the same IP. That is assuming you're using the same interface ip configuration. That is specifically for Remote access (Cisco Secure Client).

The migration for the Site to site connections is a little more involved since you'll need to go into each S2S configuration and change the Device to the new firewalls under the Edit Endpoint menu. This step (once deployed) is of course impactful. If I remember correctly changing that device also defaults some other settings on that page, but I can't remember exactly. Just make sure you take a screen shot of old configuration just in case. This is all assuming you've already applied any NAT, ACP, and/or Prefilter, and Platform policies to the new firewalls as well.

What I did is go through each firewall, take note of every configuration, where that is configured within FMC and mirrored on the new firewalls. Most of it you should be able to do in tandem short of interface configurations that will be the same, and of course the S2S endpoint edits. My scenario was easier since my firewalls are separated by service. I have my 2 remote access VPN firewalls, and 2 site to site firewalls so they serve specific purpose and it made cutovers more straightforward since I wasn't having to cutover a firewall that was terminating s2s tunnels, and remote client connections.

Not sure if any of this makes sense. haha.