No, there won't be any improvement in the visual quality.
If you are thinking about upscaling the resolution to 720p or 1080p, this would require re-encoding. If you are thinking about converting to H264 instead of the mpeg2 found in DVD-video, this would also require re-encoding, and anytime you re-encode video you will lose a bit of visual quality. If the re-encoding process is done very, very well, you might not be able to notice it without a close inspection, but it will still be there.
Most conversion programs do not have the fine controls necessary to do this well, and there is really no point in doing it.
It is possible to put most DVD-video into Blu-ray format without re-encoding. But this would keep the original resolution and the original video unchanged. Again, there is very little point in doing this unless you are making a compilation disc with several DVD's put into one Blu-ray disc.