It depends. There's an excellent chance that things won't work after the upgrade. If you use ASP scripting, you need to plan some time to test your code thoroughly afterwards. You'll almost certainly need to make changes. If you use NT4's user authentication (either local or domains), that may break after the upgrade.
The safe way to do the upgrade is to install Windows 2000 on a new box, copy your content over, configure IIS, test everything thoroughly, then swap the Windows 2000 system out for the NT 4 server.
If you don't have an extra box, or don?t want to spend the time configuring a new server, you'll need to do the in-place upgrade. If you do this, you're taking a big risk. Be sure to plan lots of downtime -- perhaps a whole weekend, if you have complex content. Of course, if all you have is static HTML and images, things will probably work fine after the upgrade.
This was first published in March 2002