You means configuration file(backup file) or?

As when you upgrade firmware, it keep the configuration unchanged and bring over, so it is not applied to it.
Unless you apply new WiFi setting, which will add the @unifi when you saving it.

That's also why sometimes prefer to clear the config/factory reset to avoid issues.

Err....
When you upgrade to new firmware, there is no new settings being patch on your old settings, it just leave it as it be, and upgrade your firmware.
The @unifi that you saw on it is just a visual one, until you click the APPLY button, it will still be the old SSID without @unifi.

Unlike Windows 10, they import your old settings when there is big update like RS1, RS2, so the old settings will be patched by new default configuration, if they has written a script for it.