> Is it really (almost?) as simple as untaring the selected tar files?  
> (being careful to save the old /etc)

Depends what you're upgrading from and to.  Whether it is sensible to
do it remotely depends on the type of hardware, console access in
single user mode, and what your fallback plan is if you break it.

I've updated machines "remotely" (aka without plugging in a keyboard
and screen for the console) but it's not something I'd want to do on a
production server I cared about.

> What happens to the shell and tar/pax that are executing when they 
> get updated?  Does tar/pax create a temp file and then do a rename?

AFAIK the old binary is unlinked and the new one written.  You could
check the source and/or use ktrace to confirm this.

Regards,

Giles