I am planning to buy an unlocked iphone 3gs and i was wondering if i can use my sim card in my current att phone which still has a 2 year contract under it.
Can i put that sim card into the unlocked 3gs with no hassels?
I do not want to jail break the 3gs, so will it and can i still upgrade it to the latest software, 3.1?
Since my current sim card does not have a data plan under it, do i have to call att and tell them i now have an iphone and add a data plan under it since i am going to use safari and browse the internet

lastly, can i sell the old att phone i used which is still under a contract?

Thanks.