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You can unlink your MyYahoo! account by going to http://my.yahoo.com

Once you log in to your old account, you can click on the "account info" link.  Click on the "Remove Corporate Yahoo"  link. That will give you the names of all the Corporate Yahoo! server IDs that the account has ben linked to. Uncheck the serverID for the CY! account you want to edit and save the settings.

This will unlink your MyYahoo account from that specific PortalBuilder server.  The next time that you view Yahoo! content on your portal page, you will be prompted to link to a existing MyYahoo! ID.