Can i buy a iPhone off the steets can connect ot with sprint?

I have a contract with sprint with my iPhone4, if i buy a sprint iPhone5 can i go and connect it an replace my 4?

From my knowledge of phones, each phone is designated one phone carrier. So if the phone off the streets was a sprint phone then yes you can switch it with your iPhone4 **IF** it has a clean ESN. That means it has been deactivated so if the phone is stollen you will not be able to use it even if it is Sprint.

As long as they are both sprint phones you just put your own sim card in and it will give you service. If not then you have to unblock it.