Reality: How good can old movies look?

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This is a very important topic to me. I've been trying to figure out how much work a studio would really need to go through to remaster an older movie from say the mid 90s and earlier to make the picture look good on the screen. Honestly can it ever really be polished? Will old movies always have grain in the background? Blade Runner will probably be the first and oldest movie to test HD remastering possibilities on old movies. After I saw what Lucas did to Star Wars it makes me wonder if they could make it looks almost flawless? Or would they have to photoshop each frame. Any information on this subject would be greatly appreciated.