Why does Yahoo ask me to login when never had a Yahoo account always an sbcglobal?

I NEVER signed up for a Yahoo account only those stupid throwaway addresses which are long gone. I’ve had an sbcglobal account since 2004 and today once again I was forced to sign in to Yahoo with my AT&T login. I presume then either AT&T or Yahoo says hey you have a Yahoo account, the one I never had with no login name or password.

From what I am to understand if I UNMERGE my so called Yahoo account from AT&T, I will likely leave behind in Yahoo my email since 2004. I clean email up regularly but there are emails with information I need quick access to so I left them on someone’s servers, wondering which. I have the feeling with the unmerging I’ll have to play a game of checking 4 accounts, with nothing on the AT&T account after unmerging, and everything on Yahoo. It makes me wonder if I'll be playing a 4 account game checking daily for which account gets my most recent email. Forwarding all the email is an INCONVENIENCE let alone having to check 4 accounts for the same person. Also worrisome is having never had a Yahoo account will I suddenly be prevented from logging into Yahoo to access old email.

How can I be guaranteed that I’m not going to be inconvenienced, that there will not be a big snafu of email lost, that I will still have access to any mail left on Yahoo, and guaranteed that all future emails come through AT&T and don’t require a 4 account daily check to find where emails actually land? AT&T has had time to work out the problems of their split from Yahoo. I feel the instructions for unmerging not fully explained, and noted those who had resulting problems dealt with unqualified technical assistance making matters worse. Looking for what others have experienced, advice, and if possible assurances that the worse is not going to happen.