I saw the latest news about the Brave as default browser on ZorinOS 17.3, but I'd like to know if it's possible to apply the Brave ZorinOS settings without a clean upgrade, in other words, I did the upgrade from 17.2 to 17.3 and installed the Brave from software app, but the ZorinOS customisation aren't available. My question is, the ZorinOS Brave customisation is only available to a clean/new installation and not with an upgrade between versions?
You will have to make those changes manually, so no. But they're only a few switches turned off in the settings, as the help article you linked to shows. Just walk through each of those, and instead of enabling features you do the opposite.
The reason this wasn't automatically applied when you upgraded from 17.2 is to avoid breaking existing installations. By installing Brave from their official repositories, you're getting the software as their developers originally intended, and not without any customized settings.
That said, you will find your changes made in ~/.config/BraveSoftware/Brave-Browser/Default/Preferences, in case you want to take this file and copy it over to another machine.
The Zorin Help item you linked to shows how to turn ON optional items that were turned OFF in the Zorin customisation, or were already set OFF as default in Brave.
I have not done this myself, but you could check out those same settings listed in the Help item, to your stock version of Brave, to turn them OFF (or confirm OFF by default) thereby applying customisation settings.
I am not sure if this other question of my own warrants a separate thread.
When I had a quick look at it on Z17.3 "Try Zorin", is it immediately asks for a Password to be created and confirmed for keyring (or something). What is that all about?
That's a thing with Chromium-based browsers. This happens when you log in automatically or in this case if you didn't provide any login details since it's a live environment. The keyring is used internally by the browser to secure information related to passwords and other details.
Choose GTK for the theme, rather than Classic or QT. This will make it cohesive with Zorin's Gnome desktop environment.
For other options, scroll a bit down. You will find a switch related to Brave Wallet, turn it off. I don't remember every single thing Zorin devs modified, but you can find them all in settings (Leo AI, Brave Rewards, etc).
If you don't want icons related to these features visible in upper right corner of the browser, just right click and select 'Hide'.