If you take the 399/499 option you're still going to be locked into a new contract beginning July 11 2008 (assuming that's the purchase date). So you'd be ineligible for upgrade until July 2010. To do what you're describing you'll have to wait for the proposed 599/699 pricing. If you're eligible for upgrade in Nov of this year then I'd go to an AT&T store on a less-busy day than July 11 and see what they can do for you. Otherwise I'd consider waiting a month or so as I've always been told my my local AT&T store that you can upgrade as long as you're within 3 months of the end of your contract.
Tried to find what I could on this, here's what I found from an ATT FAQ page, which I recall reading last year:
From:
http://www.wireless.att.com/learn/iphone-faqs.jsp
Q. Do I lose my discount eligibility on phones or devices other than iPhone?
A. No. Although existing customers are required to sign a new 2-year service contract when activating iPhone, the discount eligibility for other devices will continue from the time of your current contract. For example, if you are eligible for an upgrade discount next month, you will still be eligible for the upgrade discount on a device other than iPhone next month.
They say other than iPhone, but at the time they wrote that, there was no subsidized price. Now it doesn't make sense, within a month, their example account would be discount eligible, but only devices other than the iPhone, where's the countdown to that then, unless you are always eligible for the iPhone if you have an iPHone. And like so, with this 3G launch, all iPhone owners are eligible for the new iPhone, even if they still had 10 months left before they're eligible for other devices and just bought the iPhone last week.
Q. I just upgraded my phone recently. Are there any penalties or restrictions for upgrading to iPhone right away?
A. No. Everyone can upgrade to iPhone right away, you just need to add a Data Plan for iPhone (this may replace your current data plan but your voice plan will stay the same) and sign a new 2-year service contract. Discount eligibility for other devices will continue from the time of your current contract.
Here they specifically say you can "UPGRADE" to the iPhone right away. And that your discount eligibility for other devices will continue from the time of your current contract. Though, like the other question above, that was written before the new subsidized $199 upgrade price. So if they say upgrading to the iPhone doesn't reset eligibility, then why would the early upgrade do so. I'm only going by what ATT has posted. As there are no other details on their site about this beyond these two Q&As.