There's no bending rules to make this work. You just have to be specific. All you say to a VZW rep is that you want to upgrade one of the basic phones to a smartphone. Don't mention any switching around that you want to do until the upgrade is finished. Once, the upgrade is complete, simply ask them to put the new smartphone on your existing line and reactivate the basic phone that was just on that line used for the upgrade. This way you didn't do anything wrong, all you did was upgrade a basic phone and did a "device change" to your line with the new smartphone which in the end preserves your unlimited data. Be careful by doing this because by activating a new smart phone on your account, you will incur the $30 data plan instantly on your next bill on top of a $30 upgrade fee.
Sent from my iPhone 5S