Do you mean can you upgrade after paying off the device? I think you would still have to wait the 12 month period unless you already already met that requirement. As soon as you pay the phone (and posts to your account) and have more than 12 months, then yes you would be able to upgrade.
Payments usually post quickly when you pay with a credit/debit card on myAT&T. So you shouldn't have a problem.
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If you accelreate payments on the AT&T Next plan, pay off the device and it is posted to your account, you are able to start another Next agreement, go on a two year contract, or buy a device at full retail. There could be a few days of delay before the system sees you as being eligable for a new Next program.
The Next Plan does not lock you into a contract with AT&T and once the device is fully paid for, you own the device and would no longer be tied to the program making you eligable to upgrade or buy a new phone.
*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.