I did this once (server only to server/gateway). Here is what I did.
I SSH into the server as 'admin'. This should bring on the graphical 'Server Console' screen. Select option 2 'Configure this server' and follow the prompts. After about 7 screens you get to a screen that lets you choose server/gateway or server only. Continue on until the end, you will have to make some changes as you progress but the prompts are pretty clear. At the end you will have to reboot, and you should be golden.
If you do not know how to SSH, you can login at the server itself so log as you have a keyboard and monitor hooked up it.
Hope this helps.
-Rick Evans