...The "upgrade eligibility" date is simply the date that the customer is released from this obligation, and will again qualify for a new subsidized phone without penalty.
That above quote from the original poster is WRONG. Your contract date is 24-months from the date you originally obtained the new subsidized phone. The "upgrade eligibility" is essentially AT&T allowing you to upgrade to a newer phone before your contract end date. (which extends the time you are in contract for an ADDITIONAL 24 months from the date you upgrade your phone). It does NOT mean you are out of your obligation on that date, you are still in contract with AT&T service until the end of your contract date (which is 24 months from the date you signed up for the 2-year contract)
Now, the contract is NOT in any way attached to the phone, it's attached to the phone number. You can swap your phone to a flip phone or any other phone as many times as you want (even swapping plans and features around) anytime throughout your contract. Even if you swap phones with some other SIM enabled phone, and you switch around your plan, there is no penalty at all, granted you keep that line active on AT&T.
Also, upgrade eligibility is based on many factors, NOT on the date a phone will be coming out. Your upgrade eligibility is based on how good of a customer you are. If you have the most basic of plans and always pay late, your upgrade is at a later date. If you have a more advanced plan (multiple lines, iPhone plans, Smartphone data plans, etc.) and you pay on time, that allows your upgrade eligibility to become earlier and earlier. AT&T rewards good customers with early upgrades. Customers who constantly make payments late, constantly have their phone turned off due to late payments, constantly call in to get credits, etc. are all examples of behaviors that will ultimately keep them from getting an early upgrade.
So if you want an early upgrade, don't call in and be a complete ass to the people trying to help you, make your payments on time and when the account is updated, your eligibility date will be shortened.