You might want to research this a bit more. I've found some sites indicating you won't qualify for new customer pricing until after a waiting period:
http://forums.att.com/t5/Apple-iPhone/Canceling-or-Upgrading...
That said, if you go into an AT&T store and gently press the point, you may very well get your upgrade date moved up. It does require them calling Customer Care to ask, and they do need a store supervisor to sign off on it... but it can be done. I did it myself a few months ago, and I've heard of others doing it as well.
Be aware that AT&T ranks your value as a customer internally based on factors such as your total monthly bill size, etc. My bill is about $180-$200/month for three lines. You literally have a 'score' in their system, so your mileage may vary.
My understanding is that it is even easier to get an 'early upgrade' where you pay the discounted price + $250 for the privilege of upgrading without waiting. Early upgrades are still about $100 cheaper than buying the phone outright.
AT&T is allowing some customers to upgrade after one year instead of two if they cross a certainly monthly threshold on their phone bill. So, you may be eligible to upgrade on November 22nd, but you're still going to pay a hefty early termination fee if you canceled your account.
The reasoning behind this plan it to thwart people exactly like you (which I am).
Is it possible to use google voice such that when you make outgoing calls, your google voice number appears on the receiver's caller id, rather than your phone's "real" number?