Unable to de-couple yahoo email from sbcglobal.net account

Folks, I linked my yahoo email a while ago with my sbcglobal.net, but the latter is not used anymore.

However, I am unable to change password for my Yahoo email account, and every time I try to change, I am directed to att.com and it does not show options to change Yahoo email password. It only allows me to change password for sbcglobal.net ID.

Tried to delete the subaccount under User Profile information, Manage sub-accounts tab, but I keep getting error message PRO_UMAS_331.

Does any one know how to change? The problem is same as reported here.