Sorry, you cannot change the domain name you choose when you sign up for your plan.
If you wish to change your domain name after you sign up, you will need to cancel your service and sign up for a new plan. For this reason, we recommend that you choose your domain name very carefully when you sign up for your plan.
Not happy with your domain name? Don't worry! You can register as many domain names as you like and use them all with your plan. We suggest that you sign up for a Yahoo! Domains plan, then set up domain forwarding. When your friends and colleagues type your new domain name in their browsers, they'll simply be redirected to your current web site.
(Hint: When you set up domain forwarding, select the "forwarding to a Yahoo! hosted site" option, and your visitors will never even see your original web address.)