How to apply some configuration through Sophos Central Firewall Management

Hello, I already have 20+ XG configured and running, when I deployed them v18 wasn't released so I was managing them through local SFM but now I am updating them one by one, and now I had to move to the Sophos Central Firewall Management because of compatibility.

My doubt is, when I create a new group on Sophos Central, there is a warning saying that "assigning a firewall to the group will apply a default configuration to the firewall. It may change the current firewall configurations"

So my local configuration on the XG will be overwritten by the Sophos Central Policy?

Also, how can I deploy through Sophos Central some settings to the XGs without touching the already working configurations?

Thanks