If a bluray release is an upscale of the dvd version, then I won't bother wasting cash on the bluray if I already have the dvd version. (Alleged cases of this are Farscape, the first four seasons of the revived Doctor Who, etc ...).
Remember what I just said about upscaling. Don't ever believe anyone who tries to use that word as an excuse that it somehow makes something into HD, because it doesn't work that way. If you are watching a DVD on a high definition set, and there ISN'T a tiny postage stamp sized image in the middle of the screen, then you are already watching something upscaled - even if it looks like the standardest of standard definition DVD. All upscaling means is that the image fills the screen. That's IT.