Guild Wars 2

As found on the Knowledge Base (support.guildwars2.com):

How can I change my account name or email address?

If you have a Guild Wars and a Guild Wars 2 account, and if they are linked, you can change your Guild Wars account (log-in) name. To change your account name, please do the following:

Log into account management.
Select Edit Profile.
Enter a new and validemail in the Account Name field under the Change Account Name section.
Confirm the password associated with the account.
Click SAVE CHANGES.

Please note that if you change the name of an account that was linked to an NCSOFT Master Account, you will have two separate accounts. The NCSOFT Master Account will still be active for any other NCSOFT games associated with the account and the information you entered under Change Account Name will be your new Guild Wars account information.