Upgrading second line to Epic4g, question regarding swapping

[gobmonster]

Well, my big bro has an upgrade available but doesnt want a new phone. If i use his upgrade to get the epic4g, will sprint try to force it on his line???

If so, can i take it off the line and use it on my own??? Radio shack told me id have to wait 90 days before i switched the phone off , i havent talked to sprint yet