Active Directory – How to Migrate to New Server

The quick answer is to use dcpromo to add the 2008 server as a domain controller in the same domain that the 2003 server is in. Once the 2008 server is a DC, you can run dcpromo on the 2003 server to remove it as a domain controller. This will allow you to keep your Active Directory intact, but move the services to another server.

Also, as a general rule, it's highly recommended to have 2 servers be domain controllers (and global catalog servers) for your domain. That way your Active Directory stays up in the event of a failure (or reboot!)

There are lots of good articles from Microsoft and bloggers that describe this process. Here are some links:

2003 to 2008 Active Directory

Upgrading to Windows 2008 Active Directory

I'd encourage you to re-think the whole "rebuild" idea. There's nothing in the current AD domain that can't be "fixed" by migrating and cleaning it up.

If your decide that you have to rebuild it, you're going to need to do a PST migration of Exchange (since you're running on Exchange 2003 and can't do a cross-forest mailbox move).

Basically, this involves using ExMerge to export all the contents of the original Exchange Information Store to PST files. After you've created the new AD and new user accounts (either using ADMT or "by hand"), you'll import those PSTs back into the new Exchange IS. Finally, you'll need to use something like ADSIEDIT or a script to add an X500 address proxy to each user account that matches the "legacyExchangeDN" attribute that was assigned to their user accounts in the old domain to insure that replies to imported emails function properly (see http://blogs.technet.com/sbs/archive/2009/05/21/cannot-reply-to-old-emails-or-modify-old-calendar-items-after-pst-mail-migration.aspx for details).

Again: Re-think doing this "rebuild" versus migration. Acouple of hours with a competent AD consultant would be all you'd need to idenfity the problems in the current domain and plan for "fixes" as you upgrade to W2K8. You wouldn't need to go thru any of these gyrations with your email and you'll end up spending less time and money for what amounts to a cleaner, better solution.