What exactly can i do if i don't have a att upgrade available- iphone 3gs?

I have an iphone 3gs. If i buy an iphone 4 from a friend, can i just change the sim card and have that phone, even if i don't have an upgrade from att until april? (i know the sim card sizes are different, but it is possible to cut it down, so i would do that)

Yes, but you would need to purchase a micro sim card from AT&T in order to use the 4 for your plan. An upgrade is not needed when purchasing a phone from a third party.