Microsoft has online a four chapter domain migration cookbook. Read them if you are considering AD and migrating from NT 4.0.
- Authentication and Authorization
- SIDHistory attribute
- Effect of Upgrade on Trusts
- Using Kerberos Authentication
- Mixed Mode and Native Mode
- 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
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.