Add firewall to group in Sophos Central

I have several groups defined in Sophos Central for firewall management. Each group presently just has one firewall, and when added, I was able to import the firewall config so the device was not reset. It's working as I would expect and I can update the group policy for the firewalls and observe settings being deployed properly.

I am now ready to add a second firewall to the group, and in this case, the firewall is already configured the way I need it to be. I don't want the firewall to be wiped clean when I add it to the group. Is there any documentation or are there any KB articles that describe what specifically happens when you add a configured fireweall to an existing firewall group?

Thanks.