Regardless of what version of Windows you migrate to, make sure:
1. You know the Directory Services Restore Mode Password (if not, RESET it and TEST IT).
2. Perform a System State Backup of ALL DCs
3. If you really want to be safe, setup a test network and try this first in an environment where it will NOT affect you.
I don't have any instructions/checklists for going to 2008, but I would suggest reviewing the links below for Migrating to 2003 (and it can't hurt to keep them in mind for 2008 - there will definitely be some differences, but there should also be a good number of similarities. (Some of the links below might have been posted by Laura).
Common Mistakes When Upgrading a Windows 2000 Domain To a Windows 2003 Domain
http://support.microsoft.c
How to upgrade Windows 2000 domain controllers to Windows Server 2003
http://support.microsoft.c
[If using R2 release of Windows 2003] Extending Your Active Directory Schema for New Features in Windows Server 2003 R2
http://www.microsoft.com/d
Hotfixes to install before you run adprep /Forestprep on a Windows 2000 domain controller to prepare the Forest and domains for the addition of Windows Server 2003-based domain controllers
http://support.microsoft.c
Commodore.ca | Windows | How To Upgrade Windows 2000 Domain to Windows 2003 Server
Quote from the top of this article: "Several glossy Microsoft presenters have stated that all you need to do to complete a Windows 2003 Domain upgrade is run ADPREP and then upgrade away. This may work for very small / simple environments but it is definitely not good advice for most companies. After upgrading five servers in two unrelated domains and installing many fresh copies of 2003 I can say that I personally would not skip a single step in the process I have developed below."
http://www.commodore.ca/wi
Windows Server 2003 Upgrade Assistance Center
http://www.microsoft.com/w
Here are some links that should help you in replacing existing domain controllers and/or transferring services and Active Directory functions to another/new system:
How can I transfer some or all of the FSMO Roles from one DC to another?
http://www.petri.co.il/tra
How To Create or Move a Global Catalog in Windows 2000
http://support.microsoft.c
How to move a DHCP database from a computer that is running Windows NT Server 4.0, Windows 2000, or Windows Server 2003 to a computer that is running Windows Server 2003
http://support.microsoft.c
How can I move a DHCP database from one server to another?
http://www.windowsitpro.co
How can I move DNS from one Windows 2000 Server to another Windows 2000 Server?
(Two Related if going 2000 to 2003 - read both before moving)
http://www.jsifaq.com/subG
http://www.jsifaq.com/SUBN
Exchange Considerations:
[If you run Exchange 2000] Windows Server 2003 adprep /forestprep Command Causes Mangled Attributes in Windows 2000 Forests That Contain Exchange 2000 Servers
http://support.microsoft.c
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by: LauraEHunterMVPPosted on 2008-05-27 at 07:05:18ID: 21651968
[1] Prepare for either a 2003 or 2008 AD upgrade by following the checklist described here: http://support.microsoft.c om/kb/3253 79
com/window sserver200 8/en/libra ry/ 1d50220 9-cdb9-4e1 3-9a6c-57a d6c9d1e8b1 033.mspx?m fr=true
[2] If upgrading to 2003, follow the instructions for /forestprep and /domainprep listed in the above KB; if you are going straight to 2008, follow the steps here: http://technet2.microsoft.
[3] Add the 200[3|8] server as a member server in your 2000 domain. Once you have completed steps 1 & 2, run dcpromo to promote the first 2008 DC.
[4] Configure the 2008 DC as a GC, transfer FSMOs, modify client DNS settings as necessary.