Most people promise themselves not to upgrade but quickly give in. I upgrade stuff which is very visual (Blade Runner, Lord of the Rings) or my favorites (Tarantino, Jackson, Rodriguez, Kubrick, or just certain horror). Some people manage to control damage a bit better
If you own anything but the Final Cut of Blade Runner, that's a no brainer upgrade, or Zulu or Close Encounters of the Third Kind.
I usually refer to reviews at Blu-ray.com, blubeaver.com or DVDTalk.com.