Others feel free to comment on this point but I actually have never replaced the original .gz file and never have seen any draw backs from doing this.
We of course retain all of our originals and edited versions just in case but I've personally never had to use them.
That being said if you want to be thorough you can use 7zip which is a free utility, it has Gzip (GZ) as an option.
Just make sure both are identical files or else you might run into issues there.
Can we add a custom message like downtime notification, on the next page that we get after login which is "Site selection" dropdown , since if there is automatic sign in enabled users will directly land on the Site selection page.
No. Only if you do the customization yourself -- and it won't be supported by Tableau. Note that Tableau changes their backend so you're on your own for support (making changes) if a new version comes out and things break.
Any one tried this already? What is the exact TSM Customize syntax/command we need to use in order to show server-wide announcements?