How do I change from Windows NT to Active Directory

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I have a TS-RVHl920 using NT Domain authentication to a Windows 2003 SBS server. 
We are upgrading our Server and environment to Windows 2012 R2.

I tried changing the NT controller name in the NT environment to the new server name and also changing to  Active Directory but each time get a Fail to join the domain error.

Do I need to leave the domain and rejoin or is there a way to simple point to a new Server ?

[Texturtle]

Change back to a workgroup, then follow these steps:

http://buffalotech.com/support-and-downloads/faqs/joining-a-buffalo-nas-to-an-active-directory-domain