Bottom line is it is not a merger, it was a take over.
All the AT&T stores ONLY sell Cingular phones and not AT&T phones and plans (that's what I've found and been told). You could have called AT&T customer service and they could sell you an AT&T phone. Now you only need an AT&T phone if you are on a non-GSM account (what AT&T refers to as 'digital').
Here's what I would do. Call AT&T customer service and see if you can keep you same plan and just replace your son's phone. If they can help you out great, no changes to your existing plan and cost structure. You won't get a free phone but you keep things the same. You should be able to return the Cingular phone with no charges if it's within 30 days.
FWIW I had AT&T and switched my and my wifes phones to Cingular on a family plan because the roll-over minutes made the Cingular plan more to our advantage. It all took lots and lots of research to figure out. I've had to call Cingular customer service several times because the people at the old AT&T store botched things.