I had the same problem.  The link provided by Gent500 worked, in that it helped me change my Yahoo password! 

I then called customer service, went through about 5 different reps, but FINALLY was transferred to the "AT&T Yahoo Escalation team" who patiently worked with me to remove the link between my Yahoo account and an ancient SBCGLOBAL account that I forgot that I ever had.  The rep there, Dion, was very nice, patient, and courteous.  In 24 hours, my Yahoo account should be mine again, and no longer held captive by AT&T/SBC.  I will post again tomorrow to verify the account was really released.

So if you want your Yahoo account back, save yourself some time and ask for the "AT&T Yahoo escalation team".  When they answered, he said "AT&T advanced technical solution help desk", or something along those lines.  Don't give up!  You need to know the old SBCGLobal.net or ATT.net email address, and the service address, and your customer code (3 digits after your phone number on your bill).