One advantage of Win2K and AD is that all Domain Controllers (DCs) are essentially coequal. All DCs have a writable copy of AD and all changes get replicated to other DCs in the organization. Another advantage of Win2K (over NT4) is that you can create and/or demote DCs as needed. This is not part of the software installation as it is with NT4.
With the number of users you have, you will probably be much happier (and have less work) migrating accounts rather than recreating from scratch.The downside to this is that you will have remnants of NT4 left behind (since you will be loading Win2K over the top of NT4). If having a fresh software install is of more importance to you, it might be worthwhile to go the route of recreating yourdomain.
Another issue that might lead you to recreate vs. migrate:
When you create an AD domain (which follows the DNS domain naming convention - i.e. abc.xyz.com), AD also creates a NetBIOS domain name to be used by pre-Win2K clients and servers (in the above case it would default to ABC). With a fresh install, you can change the NetBIOS domain name to suit your needs. If you follow the migration path, the NetBIOS domain name automatically matches your existing NT4 domain name and can't be changed. In the above example, if you created an AD domain named abc.com and your existing NT4 domain is IOU, then the NetBIOS domain name automatically becomes IOU. Win2K uses NetBIOS domain names (not AD domain names) at the logon screen. This of course has the potential of creating confusion among users.
If you follow the migration path, there are 3 key things to remember:
1. Win2K DCs CANNOT exist in a NT4 domain.
2. Win2K requires AD (and DNS) to become a DC.
3. In order to migrate, Win2K (and AD) MUST be installed on your NT4 PDC.
I hope this helps with you decision process.
Good Luck!
