Help: Sbcglobal.net Email in Limbo

I have a business client who has an sbcglobal.net email that is their main email and in use for probably 20 years, used in all their advertising and registration email, etc. They moved on from AT&T over 10 years ago and at that point it was suppose to convert to a free Yahoo account. I was going to upgrade them to Yahoo Plus email so that if there were any problems accessing the account we could get support to fix it. However, I learned that it is still an active AT&T account with email hosted by Yahoo. I can log into att.com but there are no services or products associated with the email and no billing.

Years ago I had a number of clients that lost their sbcglobal.net email in similar situations where the email was not converted to the free Yahoo email after closing their ATT/SBC account leaving them in limbo. The gist of it is that if the account ever got hacked or the password needed to be reset then AT&T won't help because there is no product or service associated with the account and Yahoo can't help because it is still an AT&T account in their records.

Does anyone have any experience or advice with this or someone I can talk to? I was able to get through to AT&T support (very difficult without any account or phone number tied to the email) and they don't really understand the problem and say it will be fine but I don't want to risk losing the email.