Any new phone would have a digital serial number that is different from the IMEI on your iPhone 3. It won't have that. Pick up the phone, call AT&T, when you get a person, say 'I need to talk to someone in retentions' and firmly inform them that you got this handset over two years ago, the AT&T system is showing an upgrade, you want the account corrected to show the right date for the handset.
If they do not fix this, you're going to go down to Boost Mobile and simply have all your phones switched over.