To use your Yahoo! ID on another account you will need to delete that account and all of the information it contains before you can use that name again.

Deleting your account will also remove any other items associated with the ID such as aliases, Profiles, classified ads, and Yahoo! Mail addresses. Information from one account cannot be merged or exchanged with other accounts or aliases. Remember, if you delete your Yahoo! Mail account you will not be able to use that name again in Yahoo!