Replacing root server

First thing to do is make sure that you backup the entire system state before you do so. Next would be to make sure that the services (ie DNS, WINS, DCHP, Global Catalog,...) that this server controls are available elsewhere on the network to keep downtime to a minimum. The next thing you will have to do is find out which FSMO roles this server is holding. If it is the root of the forest, then it has at least two :Schema Master and Domain Naming Master. It may also hold three domain wide roles : Infrastructure Master, RID Master, and PDC Emulator. These roles must be available on the network. If you plan to decommission the server holding any of these roles, they must be transferred either gracefully or through various snap-ins. These roles can also be seized forcefully. I suggest that you do the research on what kind of effect this will have on your network and on your server. I have had the pleasure of doing this several times and all three ways. All have worked fine, but I really do not trust the "graceful" transfer that microsoft recommends. Please note that you will have to remove this server from the network and decommission it completely and reinstall Windows. DO NOT keep the old server running on the network after you have transferred these roles, as you risk bringing down your network.
Best of Luck

Norm