As quoted from:
http://support.microsoft.com/?kbid=292541
Although you can rename a Windows 2000 domain in some situations that are
described in this article, Microsoft highly recommends that you decide on
the Fully Qualified Domain Name (FQDN) for DNS before you actually create a
new domain or before you upgrade the domain from Windows NT 4.0 to Windows
2000. After you create the domain, you cannot rename a Windows 2000 domain
controller. Renaming the domain involves a considerable amount of work, and
it is only possible in a scenario that meets the following conditions: 1.
You have to keep the Windows 2000 domain in Mixed mode. After you change it
to Native mode, you cannot return the domain to Mixed mode, thereby
rendering renaming impossible. To determine the mode in which the domain is
currently running, expand Active Directory Users and Computers, right-click
the domain name, and then click Properties. The mode appears in the Domain
operation mode dialog box. For more information about the different modes,
click the following article number to view the article in the Microsoft
Knowledge Base:
186153 (
http://support.microsoft.com/kb/186153/) Modes supported by
Windows 2000 domain controllers
2. Because the domain is in Mixed mode, it must also either have one
or more existing Windows NT 4.0 backup domain controllers (BDCs), or
computers that are available to use as Windows NT 4.0 BDCs.
See the link for full details