This morning I decided I overreacted when I changed my email address. In the heat of the moment I took the quickest way out rather than giving the AT&T team time to work on the problem.
After all, there are a lot of smart people working at AT&T. It is possible that they will figure out what the "we've never seen this before" error message meant and fix the problem.
Then several people called to let me know emails they tried to send bounced back today. I tried to log-in to my Worldnet account to see what was happening. It was gone. Just like that. It turns out that instead of fixing the problem, they just eliminated my account.
Nice to know my instincts were right when I decided to just change my email address. Luckily, I listened to them this time.