lol
This isn't a "trick" or a "glitch". You guys are clueless. I was able to select the iPhone 5S just now and keep my unlimited data. Here's how you do it:
(1) From the My Verizon account management home screen, in the left sidebar, click Upgrade Device.
(2) Select the line you want to upgrade.
(3) When you see the iPhone you want to buy, click the name of the iPhone, not Add To Cart.
(4) At the iPhone device page, select the radio button for Month-To-Month, then click Add To Cart.
(5) Do not check the box to use your existing SIM
(6) Unlimited data is available on the next screen and it's pre-selected for you, since this is what you currently have
Complete the purchase process.
This works every single time for me. If it doesn't show up for you, then you're doing it wrong.