Although I’m not generally a big fan of “fixing” effects we now think of as substandard, there are a few cases that I think looked sub-par even when the films came out. So, like Ray Harryhausen colorizing Merian C. Cooper’s She (because the film was originally supposed to be a color film, but had its funding yanked at the last minute), I have no problem with these:
The Ten Commandments – Don’t touch most of the effects – they’re gorgeous, and not only “for their time”. The Angel of Death wouldn’t be improved by CGI, and the impressive multi-layer matting that produced the Parting of the Red Sea is still great, even if we’re so jaded now that we see the flaws.
But the cartoony animation of the Pillar of Fire and the Finger of the Lord up on Mt. Sinai doesn’t fit in with the rest of the film’s look. Even in 1956 it looked like a cartoon. Replace it with CGI-rendered flame.
Also, the bane of matte effects is the fluttering of small objects and of water droplets. You can see that some of the shots in The African Queen are effects shots because the water drops glow green. Similarly, when the Egyptian signals with a flag during the Raising of the Obelisk during the Construction of the Treasure City scene, you get annoying color separation and the flag abruptly vanishing. Fix it with CGI, and no one will even notice that you did it. But Effects are generally supposed to not be noticed, unless they’re spectacular.
The Hunt for Red October – It was early days for CGI, and they certainly hadn’t nailed down keeping things from looking non-cartoony and making things have proper shadows and not appearing to glow. Re-do every underwater CGI effect, because they look terrible. And did when the film came out. And, while you’re at it, fix the shots of Alex Baldwin and Sean Connery talking on the conning tower at the end as they move down the river – the background is bleeding through.
The Last Starfighter – I have mixed feelings about this. On the one hand, it’s a groundbreaking CGI movie. The renders I’ve seen of some still frames – where they took a lot of computing time and power – look photographically real. But they evidently didn’t have enough computing time to do the film that way, because none of the effects looks even close to real. The live-action portions and the makeup effects are still good, but it would be interesting to see how the film would look with mature CGI effects. (The filmmakers sent George Lucas a test reel, showing X-wing fighters done using their system. Lucas passed, and used model effects for Return of the Jedi, which looked very good. Years later he used CGI for his films, but only after the technology was sufficiently mature.)