That is exactly the policy that they put out when they discontinued the unlimited plans. At the time, people were complaining that their plans switched even though they were adding an off-contract phone. It turns out that there is subtle distinction between adding or activating a phone and changing the device. It is (probably by design) very easy to overlook and complete the process without realizing what you have done. Once done, it can't or won't be undone.
Originally Posted by CHIP72
Fortunately, when I switched from a Thunderbolt to a Rezound, I simply moved the SIM card with no trouble. I love 4G.
Now there is a new caution. Unlimited customers are being invited the join their Edge program. This allows one to upgrade phone more frequently. What they bury is that this requires a switch to a limited plan. So watch out!
I'll probably give up my unlimited plan when they move to the next generation data network. For us, it's not as much as a loss. Our unlimited plans are $70/mo. Since they're not shared, we have two. If we had $30 unlimited, it'd be a different story.