I did find two threads on the Verizon forums, where Customer Service said that the promotional credit stays with the line and not the device. As long as the payment agreement is continued, it doesn't matter what device is on the line.
This makes sense, of course that is why I don't trust it - Verizon is always looking to make money if they can.
If your phone gets stolen/broken/lost before the 24 months, you can use an old phone on the line and not lose the promotional line credit is what they said.
If you try to purchase another phone with a payment agreement for that line or cancel that line, you would lose the credit.
Anyone with first hand experience on this issue ?