Originally Posted by HarryT
Very true, but I would suggest that reflashing this device with the normal Kindle ROM image would be copyright infringement, given that the unit you've purchased isn't licensed to run that firmware.
Flashing a firmware image to a device that you own is considered "fair use" in many countries. Public distribution of that image without permission from the copyright holder IS copyright infringement.
In general (where I live) you can copy anything for your own use, but you cannot sell or distribute your copy. It gets complicated with encrypted stuff, because cracking the encryption to modify the image can violate the DMCA, but using an unmodified encrypted image is okay (as long as you do not distribute copies of it).
The kindle images are not encrypted, so for personal use you can modify them and use them on your own devices.
The "legal" way is to copy an image from another device that you own (or borrow a donor device from a friend). I did that to debrick one of my K4s with an image from my other K4. I had problems registering it with amazon, but a tech at amazon registered it for me (which somehow made the special offers go away).
The only problem I see with that on your demo DX is the serial number may be registered at amazon as a demo device, and that serial number may not be allowed to use 3G until you register it. Registering it may require technical assistance from amazon. You may need to "talk nice" to the amazon tech to achieve success.