If the SIM Card is from the same cell phone Carrier, you can switch SIM Cards whenever you like. If your iPhone is unlocked for all Carriers, then you can switch SIM Cards from any Carrier whenever you like. iTunes is not necessary.
thanks for replying but the thing is how do i know if my phone is unlocked for all carriers and i'm actually not from the us. so does that make any difference whether which country im in? also if i'm changing cell phone carriers i'll be given a new sim card. is there any solution for this? thanks!