Why would you even buy an iPhone 5 if you have a 4 at home?
Regardless, to get an iPhone for $200 you're clearly talking about signing a contract. If you sign a contract, you are legally obligated to follow through with it. That contract states you will use the phone that you got when you signed it. The only way out is to buy out the contract, and it's incredibly expensive. $40 per month left of the term in fact. So if you sign a 3 year contract, that's $1440.