Often, there comes a time where you need to replace an existing Domain Controller, with a completely new box running new hardware. This can seem like a challenging and difficult task involving lots of downtime, however, this is a misconception and the task itself is actually fairly simple.
 
Detailed below are the steps for replacing a Domain Controller with the least administrative effort and almost no downtime. These steps are aimed at standard and enterprise editions of 2000, 2003 and 2008 Server and not SBS. SBS is a completely different kettle of fish.

			1. Preparing your domain with the ADPREP Tools

Adprep /Forestprep and /Domainprep are tools available from Microsoft which are used to extend the windows 2000 schema to cope with 2003 Functionality. 2003 Introduces a whole new set of features and functionality which are foreign to the Windows 2000 platform.
 
Adprep /Forestprep Needs to be run on the Schema Master and /Domainprep need to be run on the Infrastructure master (If a single DC environment then this obviously be held on your only DC). Forestprep is run once and will propagate its changes throughout the forest, and Domainprep needs to be once per domain. If you are in an environment that contains child domains, then you will need to run Domainprep on one DC on each domain.
 
The Adprep tools can be found on the I386 folder of your windows 2003 CD
 
Server 2003 R2 also has an extended schema, which is foreign to both Server 2000 and Server 2003 Release one. The ADPREP tools again need to be run to extend the schema, but this time they need to be run from the second CD in the R2 set, and they need to be run from the \COMPNENTS\R2\ADPREP directory.
 
More Info on the 2003 R2 Schema Modification
http://www.microsoft.com/downloads/details.aspx?familyid=5B73CF03-84DD-480F-98F9-526EC09E9BA8&displaylang=en
 
What’s R2 all about?
http://www.microsoft.com/windowsserver2003/r2/whatsnewinr2.mspx
 
Of course with the introduction of Windows Server 2008, there is a new generation of ADPREP tools that need to be applied.
http://technet2.microsoft.com/windowsserver2008/en/library/1d502209-cdb9-4e13-9a6c-57ad6c9d1e8b1033.mspx?mfr=true

			2. Adding your new Domain Controller

Join your clean install of 2003 to the domain the same way you would a workstation or client machine. This makes it a member server. (Right Click My Computer, Computer Name, Domain)

On your current Domain controller, make your DNS zones Active Directory Integrated. This allows for DNS to be replicated with Active Directory as discussed in the next paragraph, giving you a complete replica and a redundant set of zones in case of failure.

Now that you have completed the ADPREP tools, you can now promote your new server to a Domain Controller using the DCPROMO wizard. When running the wizard, you need to select the option to “Add as an additional domain controller in an existing domain”. Add your appropriate credentials and follow the wizard.
 
This process will replicate your Active Directory structure to the new server, giving you two DC’s running on your domain.

			3. Transfer the FSMO roles

Every Forest contains 5 FSMO roles which are crucial to the operations of Active Directory.
 
Schema Master, Infrastructure Master, PDC Emulator, RID Master, & Domain Naming Master
 
These roles by default are held on the first Domain Controller in the forest however, they can be dispersed amongst multiple DC’s in the Domain.
 
I mentioned that there are 5 roles. In an environment where you run child domains, this statement is true yet slightly different as well. Every domain within the forest contains its own set of PDC Emulator, RID Master & Infrastructure Master roles. The Schema and Domain Naming Master roles are held only on the root domain.
 
An example:

You can query Active Directory on any domain controller to find out where the FSMO roles are store by using the “netdom /query fsmo” command
 
When removing a Domain Controller from the domain it is best practice to transfer the FSMO roles to another DC before using DCPROMO again to demote and remove the old DC. For the record, the DCPROMO wizard is indeed designed to move the roles automatically when demoting a DC, but it’s not a very reliable tool and the manual approach ensures that you have no issues at demotion time.
 
A guide on transferring the FSMO roles:
http://www.petri.co.il/transferring_fsmo_roles.htm
 
http://support.microsoft.com/kb/255690

			4. Make the new DC a Global Catalog

The global catalog basically holds a partial replica of every object within the Active Directory forest. Without the GC, you would not be able to search the directory efficiently, especially in Forests that contain multiple domains. With a Global Catalog, you don’t need to know where the object physically resides as the GC holds that information automatically for you.
 
You need to ensure that there is at least one GC per site (in multi site domains) and that if replacing a DC, you make the new DC a GC.

How to create or move the global catalog in windows 2003:
http://support.microsoft.com/kb/313994

			5. Dealing with DHCP

Deactivate DHCP on the old DC (if used) and recreate the scope on the new DC, note if you have a fairly complex or large DHCP scheme you may want to export and import the database
http://support.microsoft.com/kb/325473/

			6. DCDIAG, NETDIAG and a healthy domain

DCDIAG is a crucial tool for diagnosing and ensuring there are no issues with your active directory infrastructure especially in multi DC environments. It can be found in the resource kit of downloaded direct from Microsoft. Run DCDIAG and make sure that there are no errors. If there are errors then you should resolve them before continuing.
 
DCDIAG
http://technet2.microsoft.com/windowsserver/en/library/f7396ad6-0baa-4e66-8d18-17f83c5e4e6c1033.mspx?mfr=true
 
NETDIAG is another crucial diagnostic tool which focuses more on the network side of things and the communications between sites
 
NETDIAG
http://technet2.microsoft.com/windowsserver/en/library/cf4926db-87ea-4f7a-9806-0b54e1c00a771033.mspx?mfr=true

			7. Recreate Shares on the new DC

There are a couple of ways of doing this depending on the complexity of your structure. Often if it’s a small environment it’s easiest to create the shares again and editing logon scripts etc to point to the new DC.
 
You can use a backup program to backup shares and restore them to a new location
 
You can also use robocopy from the resource kit to copy the data in bulk or there are options such as Teracopy to allow you to do this via a GUI interface.
http://www.codesector.com/teracopy.php
 
The choice is really left up to you and the nature of your structure

			8. Reinstalling and sharing printers

This is a pretty self explanatory step…however if you have a large amount of printers installed you may want to have a look at the printer migration tool from MS
 
http://www.microsoft.com/downloads/details.aspx?FamilyID=9B9F2925-CBC9-44DA-B2C9-FFDBC46B0B17&displaylang=en

			9. Client Based Changes

Once your replication and migration of Active Directory has occurred, and you have pulled your data across to the new server, You will need to change the DNS settings at a client level. If you are running DHCP these changes are easy to make, but any statically assigned devices will need to have the changes made manually. You now need to point the client machines to the new Server for DNS. Depending on the choice you make in the below step, you may also want to leave the old server as a secondary DNS entry. Ensure that there are no External DNS server entries on your client machines, any external DNS entries should be configured as forwarders within DNS

			10. Demoting your old DC

Now that you have confirmed that your new server is now acting and functioning as a DC, you have transferred the roles, confirmed that DNS has replicated, ensured it is a Global Catalog Server and have transferred the FSMO roles, it’s time to demote the old server.
 
Run DCPROMO on the old DC and select the option to remove AD from this machine. Follow the wizard and when complete reboot. You have no removed Active Directory from this server and it is no longer a Domain Controller.
 
For the record, It is always best to have more than one DC. If you are replacing the DC because it is old and due for replacement, but the machine itself still functions, then I heavily recommend using this Server as a replica DC. A format to clean it up and then adding it back in as an additional domain controller gives you redundancy and a failsafe in case of failure. It doesn’t have to contain any other roles besides that of a Domain Controller and DNS server.