I will be dealing with this myself when I make the move to Ultra this coming Xmas.
I looked into it a bit, and doesn't seem to be a definitive easy answer.
Most of what I have seen is what I figured... its a huge jump from 480 to 4K. With the original resolution being so small in comparison, it's just not going to look perfect.
Also, the source dvd is apparently a factor as well. DVDs that were manufactured earlier vs ones produced recently will not look as good. Back when they had non-anamorphic dvds, interlacing or not, etc.
So to me, that means if you want the video to look EXACTLY like it was supposed to, you have to stick with the original resolution, without any alteration or upscaling- a regular 4:3 TV and dvd player (or blu ray player, manually set to output at 480 and 4:3 (fullscreen).
Sacrificing a bit of detail and seeing some artifacts... you could take your step up to a 720 TV and still be happy with the picture.
I have mine going at 1080 and the picture looks good to me. Its not perfect, but still good. Again, the larger you go, you will see more artifacts and imperfections.
There is something to considering the source... older film/tape quality, has there been any remastering done...
I've seen some posts and articles that say the video processor that does the upscaling (either on the TV or blu player side) is a factor. Not all are created equal... lower quality/ cheaper ones wont do as good a job as higher end upscalers- so it may depend what you have hardware wise.
I can't tell which way the graphics load should be. Some say to turn off all upscaling on the player and let the TV handle the upscaling by itself, some say it should be the other way around.
I have a friend that just went to Ultra, I asked him how standard dvd playback was and he said it was fine. Id have to see if I have any Region 1 DW dvds to try on his system... all my DW are the UK releases.