IPhone 4S to iPhone 5S SIM Transfer?

I have an iPhone 4S now with Sprint and it's not time for my upgrade (2 years left) So can I buy an iPhone 5S from apple and just take out the SIM in my 4S and put it in the 5S? Would I need to activate it or would I still need an upgrade?

The iPhone 5S has a different SIM card so you'll need a new one, but I think you can get a iPhone SIM card with the number you already have and turn yours off. I did that at AT&T when I switched back to my iPhone 5 from a Galaxy S4.It cost $20 though.