iPhone exchange

[jjmontiel82]

Last night my car got broken into because I forgot to take out my iPhone.
We have 3 lines on my plan, 2 lines have iphones (including the stolen phone) and third has an old flip phone.
The person with the flip phone told me that I could get the iPhone using their line because Sprint charges me the full price of the phone with my number.
If they get the phone on their line, can I get their new iPhone and they keep their flip phone?
How can it be done? Do I have to do it in the Sprint store and it be done immediately after purchasing the new iPhone?

Thanks in advance