Microsoft has online a four chapter domain migration cookbook. Read them if you are considering AD and migrating from NT 4.0.

Domain Migration Cookbook - Chapter 1: Security

• Authentication and Authorization
• SIDHistory attribute

Domain Migration Cookbook - Chapter 2: Domain Upgrade

• DCPromo.exe
• Effect of Upgrade on Trusts
• Using Kerberos Authentication
• Mixed Mode and Native Mode

Domain Migration Cookbook - Chapter 3: Domain Restructure

Domain Migration Cookbook - Chapter 4: Restructuring Tools

• Active Directory Migration Tool : MMC snap-in consisting of wizards that perform various migration tasks. Microsoft licensed this migration technology from Mission Critical Software and make it available free.
• ClonePrincipal: The ClonePrincipal User Guide, Clonepr.doc, is located in the Support.cab file in the support\tools folder on your Windows 2000 CD
• Netdom
• MoveTree

There are good third party migration tools available. See NT/W2K Domain Migration Utilities

On your Windows 2000 CD in the support.cab is another Microsoft migration document, Planning Migration from Windows NT to Microsoft Windows 2000 (Domming.doc).

There is a lot of confusion about mixed mode versus native mode domains. A mixed mode domain can support NT 4 BDCs. Once all NT 4 domain controllers are converted, you can change the domain to native mode. There is no going back unless you left an NT 4 BDC offline. In any case, mixed mode and native mode pertain only to the domain controllers in the domain. A Windows 2000 domain in native mode will support legacy clients including Win9x, NT 3.51 and NT 4.0.