How to Change an ID Name in Yahoo!
Each Yahoo! user has a unique ID name to access Yahoo! features such as email, instant messenging and games. You cannot change your ID name; if you want a new ID, you have to start a new account. However, you are allowed to create an alias name (up to six aliases per account), which you can display to other users so they won't see your Yahoo! ID.
Click your ID name in bold at the top of the Yahoo! home page. You may need to sign in to see the link. Click "Account Information." If Yahoo asks for your password an additional time, enter it. This is for security purposes.
Click "Account Settings" near the bottom left side of the account information page and then select "Manage Your Yahoo! Aliases." This brings up a new page for you to create and manage your aliases.
Click the link that says "Add an Alias." In the text box, type in your desired alias name and click "Save." Occasionally, someone may have already registered an alias you wanted, in which case you will have to think of something else.
Use an alias by clicking "Edit" in your Yahoo! messenger window and selecting the alias you want to use. Selecting an alias puts an "X" next to the name to show that it is active. To message a friend with your alias, double-click her user name in your friends list. At the top of the messenger window, select the alias from the drop-down box.