Please note that while it is possible to change your account subdomain, existing pages will not forward to the new account subdomain.
Any users who click on ads or links that you have that direct traffic to your current account subdomain will receive a broken page error message.
To change your account subdomain, go to My Account > Account Details and type in the new subdomain.
Click Update Account Settings
You will also need to make sure that all of your account information is filled out completely prior to updating the settings.