It is Samsung's site.
I talked with Sprint, and this is what they said.
7:06:26 PM : (Me): Thank you, but I was wondering that if I wanted to upgrade for a price of $xx.xx (which it says requires a new service plan), will I HAVE to get a new service plan?
7:07:14 PM : (Me): Or do I just choose the plan I am currently on when ordering, and activate the phone myself when I recieve it?
7:08:29 PM : Dolly K: If you are upgrading the phone with existing line , you need to opt your current plan which is already activated on your account.
7:09:04 PM : Dolly K: Once you will get the new phone, please contact us via chat and we will activate the phone on your account with existing plan.
7:09:47 PM : (Me): So I don't have to worry about it saying that I require a new service plan?
7:10:04 PM : Dolly K: Please do not worry about that.