you can change the name on your account information. go to att.yahoo.com/membercenter website --> login using your email ID --> click on MY ACCOUNT AND BILLING tab --> under Personal Information, click on VIEW/EDIT CONTACT INFORMATION, MARKETING PREFERENCES, ETC. link --> on Yahoo! Address/Contact Information, click on the EDIT button --> you can change your name on the next screen --> click on SAVE CHANGES button (bottom-right corner). logout and log back in, see if changes the display name.
I apologize for the inconvenience which you are experiencing.
The information which you are talking about (welcome name), is an editable field on your AT&T email account and if anyone knows your email address and corresponding password, that person can edit or change that (welcome name) information.