iphone 2g to iphone 3gs! change data plan? HELP fast!?

I currently have an iphone 2G and i want to switch to an iphone 3gs. I'm already "grandfathered" by my old unlimited plan, but if I get an iphone 3gs, does it affect my contract?
in other words, can i buy JUST the iphone 3gs?!
my boyfriend has an iphone 3g that he is willing to let me have.
will going from 2g to 3g affect my contract?
 
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