Can I change my Yahoo! ID?

Your Yahoo! ID is unique to you and cannot be changed. The only way to change your primary Yahoo! ID is to create a new account. If you decide to create a new Yahoo! ID, you need to recreate all of your preferences for My Yahoo! and other personalized Yahoo! services.

Even though you can’t change your ID, you can have up to six aliases for your ID. An alias is a nickname you use instead of your Yahoo! ID in places like Yahoo! Messenger and Yahoo! Message Boards. For example, you could create the nickname “PokerPlayer” and use that alias to identify yourself when you join a group.

An alias allows you to remain anonymous. You can interact with others without sharing your Yahoo! ID, which is also the prefix of your email address. You must log in to Yahoo! with your unique Yahoo! ID, but once logged in, you can choose any of your aliases.

Source: yahoo help