IPhone 4s to the iPhone 5 upgrade while on a plan?

I've herd that if you have a iPhone 4s still on a plan (which i do) you can upgrade it to a 5. Like hand it in pay a bit extra and receive a 5 and just pay of that? Is this true? How does it work and how much? I would love to upgrade to the 5 but my 4s is on a plan still and i'm still paying it of, can i still upgrade?

Apple releases a new phone every year so they can capitalize on people buying the new product. Cell providers put up with it because people pay early termination or add new lines just to get the phone.

You'll have to either pay full retail/off-contract prices or pay to terminate your contract early in order to start a new contract with new phone prices.