Your network contains an Active Directory forest.
You add an additional user principal name (UPN) suffix to the forest.
Youneed to modify the UPN suffix of all users.
Youwant to achieve this goal by using the minimum amount of administrative effort.
What should you use?

A.
the Active Directory Domains and Trusts console

B.
the Active Directory Users and Computers console

C.
the Csvde tool

D.
the Ldifde tool

Explanation:
!***
Old answer: the Ldifde tool
http://technet.microsoft.com/en-us/library/cc772007.aspx
Add User Principal Name Suffixes
You can use Active Directory Domains and Truststo add user principal name (UPN) suffixes for the existing
user account. The default UPN suffix for a user account is the Domain Name System (DNS) domain name of
the domain that contains the user account. You can add alternative UPN suffixes to simplify administration and
user logon processes by providing a single UPN suffix for all users. The UPN suffix is used only within the
Active Directory forest, and it is not required to be a valid DNS domain name.
To add UPN suffixes
1. Open Active Directory Domains and Trusts. To open Active Directory Domains and Trusts, click Start ,
click Administrative Tools and then click Active Directory Domains and Trusts .
2. In the console tree, right-click Active Directory Domains and Trusts and then click Properties .
3. On the UPN Suffixes tab, type an alternative UPN suffix for the forest, and then click Add .
4. Repeat step 3 to add additional alternative UPN suffixes.
Additional considerations
..
You can also perform the task in this procedure by using the Active Directory module for Windows
PowerShell.
Community Additions
Sample AD PowerShell command to update UPNs in bulk:
Get-ADUser-Filter * -properties homemdb | where {$_.homemdb -ne $null} | ForEach-Object ($_.
SamAccountName) {$CompleteUPN = $_.SamAccountName +”@contoso.com”; Set-ADUser -Identity $_.
DistinguishedName -UserPrincipalName $CompleteUPN}
The above script:
Gets all users with something in their homemdb attribute (i.e. mailbox users)
Creates a temporary variable called $completeUPN which is a combination of every user’s
samaccountname plus @contoso.com
Sets each user to this new upn