You want to rename a domain due to organizational changes or legal restrictions because of an acquisition. Renaming a domain is a very involved process and should be done only when absolutely necessary. Changing the name of a domain can have an impact on everything from DNS, replication, and GPOs to DFS and Certificate Services. A domain rename also requires that all domain controllers and member computers in the domain are rebooted!
Under Windows 2000, there is no supported process to rename a domain. There is one workaround for mixed-mode domains in which you revert the domain and any of its child domains back to Windows NT domains. This can be done by demoting all Windows 2000 domain controllers and leaving the Windows NT domain controllers in place. You could then reintroduce Windows 2000 domain controllers and use the new domain name when setting up Active Directory. The process is not very clean and probably won't be suitable for most situations, but you can find out more about it in MS KB 292541.
A domain rename procedure is supported if a forest is running all Windows Server 2003 domain controllers and is at the Windows Server 2003 forest functional level. Microsoft provides a rename tool (rendom.exe) and detailed white paper describing the process at the following location:
The domain rename process can accommodate very complex changes to your domain model. You can perform the following types of renames:
Rename a domain to a new name without repositioning it in the domain tree.
Reposition a domain within a domain tree.
Create a new domain tree with a renamed domain.
One thing you cannot do with the domain rename procedure is reposition the forest root domain. You can rename the forest root domain, but you cannot change its status as the forest root domain. Another important limitation to note is that you cannot rename any domain in a forest that has had Exchange 2000 installed. A future service pack release of Exchange Server 2003 will reportedly handle domain renames. See the web site mentioned in the solution for more information on other limitations.
MS KB 292541 (How to: Rename the DNS name of a Windows 2000 Domain)