Is it possible to use yahoo true switch to merge an additional yahoo email account? Can a user name be changed

No, Trueswitch is used to transfer data from one account to another when you switch accounts and it does not support y! to y!. What you can do is set your one mailbox to fwd messages to the other. That requires though that the receiving mailbox is Mail Plus.

You can create an alternative email address by going to Mail Addresses at the mail options page and create another email address associated to the same mailbox. To change your actual name, go to general preferences on the mail options page

hope it helps

i am not sure about the first part but you can change your user name.