Did you pay a reduced price for your new phone? If so then you most likely renewed your contract. If you paid full price and already had a line with them then most likely they renewed it anyway because they can but you can get out of it by just calling and making enough fuss.
Well here's the situation. I didn't get the subsidized price because I wasn't eligible for another 4 months or so (sept) but now I want to sell my unlocked phone for 800 and buy it back at 300, I assumed my eligibility was still Sept 09 but since I signed a new contract, it's not until 2011. Can I demand for the subsidized price?
No you probably can not. You might not have gotten the brand new contract prices, but you probably did get the exception pricing. It's not as cheap as a full upgrade (because you're not within that time frame) but you should have gotten a discount of some sort off the no commitment price ($399/$499 for the 16gb/32gb). No commitment was still higher than that, but wouldn't have required a new contract. If you paid the $399 or $499, you did get a discount and renew your contract.