Currently the word is that you will keep the unlimited plan if you upgrade from a 3G Smartphone (the Droid X) to a 4G LTE device.
Effective July 7, 2011:
* If you have a Smartphone with unlimited data and are changing to another Smartphone, you are able to keep unlimited data.
*If you change from a 3G Smartphone with unlimited data to a 4G Smartphone, you are able to keep the unlimited data.
* However, if you change from a Feature phone or Multimedia phone with an unlimited data plan, to a Smartphone, you will have to choose from one of the new data plans.
~via ChristinaB_VZW, Verizon Employee