can i use my smartphone's SIM card in an iphone without chaning the bill?

okay so i have an htc aria by att and my mom has an old iphone 3gs that she is willing to let me have. can i use the SIM card from my HTC Aria, in the iphone without causing any changes to the bill? oh yeah both the htc and iphone are from at&t
thanks
how can i go to jail? we bought the phones, they're ours.
Thanks Kyle