You want users to log on to Active Directory by using a new Principal Name (UPN).
You need to modify the UPN suffix for all user accounts.
Which tool should you use?

A.
Dsmod

B.
Netdom

C.
Redirusr

D.
Active Directory Domains and Trusts

Explanation:
http://technet.microsoft.com/en-us/library/cc732954%28v=ws.10%29.aspx
Dsmod user
dsmod user -upn <UPN>
Specifies the user principal names (UPNs) of the users that you want to modify, for example,
Linda@widgets.contoso.com.