Switching from an iPhone 4 to 4s

[bradwing15]

I plan on switching from an iPhone 4 to a 4s for many reasons. Anyway, that's not what concerns me. I am involved in a plan already, would I have to pay the ridiculous price of $500+ for the new phone or, because I'm already in a plan, would I be able to switch phones and pay the $199/$299? Or would I not be able to even switch within my plan?