If I already have at&t, can an unlocked iphone 4 replace my iphone 3gs on that same contract?

I currently already have a contract with at&t, and I have an iphone 3gs. I want an iphone 4, and I'm considering buying an unlocked one off of apple.com. I'm wondering if I can do this and continue my current contract (which is only about 8 months in, it's a 2-year contract) without having to sign a new one? I know that the 3gs and 4 take different SIM cards, would if be possible for me to just get a new SIM and transfer my info on it in the store and put it in my iphone 4 while still continuing and keeping my current contract/number? Or is it required to get a new contract with an unlocked phone like that, even if I already have an ongoing contract with at&t?

Thank you!