Microsoft: Windows & Exchange Changing an Username and Email Address

Step 1 - Change the Username, Logon Name and Display Name
1. On the Exchange server > Start > Run > dsa.msc {enter} > Locate the user in question > Right click and rename.

Note: You can do this on any Domain Controller but for Step 2 we will need to be on an Exchange server, or a machine with the Exchange Management tools installed.

2. As soon as you press {enter} > The rename user dialog will open, and you can change the display name, and the user logon name > OK.


Step 2 - Exchange 2000 / 2003 (Including SBS 2000 / 2003) Change the Email address
Note: For newer versions of Exchange see below.

1. Whilst still in active Directory Users and Computers > Right click the affected user > Properties.

2. E-mail Addresses tab > Add > SMTP Address.

3. Type in the new email address > OK.

4. Untick the "Automatically update email..." option > Select the NEW email address >Set As Primary > Apply > OK.

Note: It can take a while for your global address list to update, then your Outlook clients need to get the updated list, sometimes this can take a couple of days! Be patient, the changes have been made.


Step 2 - Exchange 2007 / 2010 (Including SBS 2008 / 2011) Change the Email address
1. On your Exchange 2007/2010 Server Launch the Exchange Management Console > Recipient Configuration > Mailbox > Locate the user > Properties.

2. E-mail Addresses tab > Add > Type in the new address > OK.

3. Untick the "Automatically update email..." option > Select the NEW email address > Set As Primary > Apply > OK.

Note: It can take a while for your global address list to update, then your Outlook clients need to get the updated list, sometimes this can take a couple of days! Be patient, the changes have been made.


Reference:
Changing a Username and Email Address (Windows & Exchange)
http://www.petenetlive.com/KB/Article/0000698.htm