Switching upgrade iPhone to another Line
So I have a Sprint Family plan with my parents and sister. My mom had an upgrade and let my sister use it to get the iPhone 4S. I have read that Sprint wants a newly upgraded phone to stay on the eligible phone line for up to 30 days before you can move the new upgraded phone (iPhone 4s) to another line. Right now the iPhone is activated and still on my moms line. Does anyone know the specifics as to when I can switch the iPhone to my sister's line without incurring any additional fees?
Thank you very much.