Yeah, I was saying "reboot" only because you'd have to do some trickery with a cron job or sleep command to get the "ifup" to run shortly after you do "ifdown", because as soon as you do "ifdown" you'll loose your network interface, and therefore your SSH connection will die, meaning you'll not have any more control over the box at that point. You'll have to have some independent process on the box which will fire off a few seconds later to bring the interface back up with the new config.
You're right, this is not NT. But it's also not XP, where you can change your network settings inline without even the network dying... I've literally been connected to chat or email clients, and changed from one static IP to another, and not lost any connections (though I admit I did loose my SSH client connection to one of my servers, duh..)