This is my experience from AT&T. I pre-ordered the iPhone 5 off of another line's eligibility so I could use it with my own number. It worked. The entire process was relatively painless. I called AT&T's customer service and asked I asked discreetly to waive the activation fee, and make sure that the 300MB plan be cancelled on the line I originally meant to go upgrade. The packaged SIM worked right out of the box. My unlimited data plan was perserved. I bought a Galaxy S III a month prior. Overall, I was a bit impulsive and got caught up in the the pre-order hype. I figure, in the change I wasn't going to keep both phones, I'd return or sell + keep the other. I return the iPhone 5. I have a nano SIM that I couldn't easily use in an iPhone 4 or a Galaxy S III. I went to a corporate AT&T Store where they activated a new SIM and attached the Galaxy S IIII IMEI to it. I was asked and printed out a receipt with a description of changes (unlimited iPhone to unlimited lte). Through e-mail and through a receipt that was given to me, they changed my data plan to a 300MB $20. I didn't authorize. I didn't know about it. I got no warnings on any of my devices. Late in the week, my unlimited plan was deactivated and this new plan was put on in its place. There modifications were "explained" as to why they kept changing my unlimited plans to other unlimited plans until they settled at the 300MB cap.