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If you've got a low resolution scan you can "rezz it up" increasing the weight of the scan--but you won't actually increase the detail. Consider that you've got a computer that sees your image as a stack of multicolored bricks. Instead of building your wall with 72 bricks for each inch, you've asked it to use small bricks. But the number of colors or fine details can't be arbitrarily "imagined" by a nonartistic computer. Yet. Though some fractal-based software is showing some hope.
Now if you can lay your hands on the original artwork and scan it at a higher resolution, you preserve the detail. You can even make another copy and "rezz it down" preserving more detail than an original, coarse scan.