Can I switch an Iphone from C Spire to AT&T?

I bought Iphone 4s from an individual. He used C Spire and I use AT&T. What do I need to do to get C Spire off the phone. I know I have to take sim in to AT&Tto make it work but do I need to deactivate C Spire first?

C spire is a CDMA network that does not use sim cards. However, the phone may be locked and you may not be able to use it with a GSM carrier until it's unlocked. You're only going to find out by putting a sim card into tne phone and seeing whether it works. If it doesn't, it's likely that only C spire would be able to unlock the phone and it's doubtful that they would. Also, since that "individual" probably did not complete his contract with C spire, the phone can't be used with C spire.