The only way to change your Yahoo! ID is to open a new account. However, you can have up to five aliases as part of your current Yahoo! ID/account and use these aliases to chat.
An alias consists of a name and some information that you show in a profile to other Yahoo! users.
Create an Alias
To create a new alias, just click the "add/change profile" link on the top of the Welcome page. The next page lists your ID and any aliases that you have already created. To make a new one, click the "Create New Public Profile" button near the top. The only required field is the actual name of your alias; all other information is optional. Do not reveal any information on your profile that you wouldn't want a complete stranger to know (phone numbers, addresses, etc.).
When you're done, click the "Create Profile" button to return to Yahoo! Chat, which now includes your new name on your alias list.
Edit Alias Information
You can edit the profile information attached to an alias before you enter Yahoo! Chat or at any time while chatting.
To edit the profile, click the "add / change profile" link on the Welcome page. On the next screen, use the "Edit" link next to the alias you would like to edit. When you're done, use the "Click Here When Done" button. Then click the "Finished Editing" link to save your changes and return to Yahoo! Chat.
Select a Default Alias for Chatting
For easy reference, the default ID that you have currently selected to use is always listed in the Welcome line at the top of the Yahoo! Chat front page.
If you would like to change to a different default alias under the same ID, click the "add/change profile" link. This takes you to a separate page with a list of all of your available profiles. In the list, locate the alias you would like to use, and click the "Make Default" link to the right.
Click the "Finished Editing" link when done to save your changes and return to Yahoo! Chat.