They (TVandNAV2go) just add the inputs, no need to change the screen. It's kind of expensive ~$400 + shipping. It is possible to also modify you VES system to have a video out.I dont think things are as simple as that. I dont believe our NAV screens are set up to be able to play DVDs period. There would be some serious wiring adjustments needed to make this happen, but in the end I honestly dont think it would work.
If you want your stock NAV unit to play DVDs, I would suggest looking into what the company TVandNAV2go does. You mail them your stock unit, they change around the internals (and probably the screen itself to a better quality one) so that it can play DVDs and accept a back-up camera and then send it back to you. The turnaround time is about a day if I remember correctly. Worth looking into at least if you want to keep that stock look but have the DVD playback feature available on the front radio.