Yea I called att as they told me thisi'm doing this too. im taking my moms upgrade so what i'm going to do is take out the SIM card of the iphone i have now, put it in the new device.
i'll take the new sim, put it in my old device (which is now my moms new device), and activate it that way.
basically i'm swapping the SIMS. I'm not going to turn the new phone on until i have MY sim in it so it doesn't activate her number on the new SIM.