I have an iphone 4 and I want to know if I can change/ swap for a iphone 4s?

I have an iphone 4 and I want to know if I can change/ swap for a iphone 4s?

Added (1). I'm on pay as you go

You can trade your iphone 4 in and then buy the iPhone 4S… Why not get the iPhone 5?

Call your carrier, if you are still on contract then you probably have to buy it on your own. They aren't picking up the cost for you just for the heck of it.