If one would update the graphics of the game, that would effectively require re-creating most of the art resources of the game. This is often the most costly area of modern game development, as it requires (initially unknown number of) artist hours.

In addition, if the game doesn't have an open, "moddable" architecture, it is often difficult to integrate art changes to it. Commonly, games tend to use proprietary packing formats to store their resources in order to optimize their loading and obfuscate the assets. I don't remember whether or not this is the case with the particular games in question.

I doubt it with those particular games. (I could be totally wrong, however it's not likely that modders could improve upon what the pro artists did IMO. I've seen it happen, but it's not typical.)

If you're really interested in doing this, I would seek out a modding community forum for those particular games. You're really talking about "modding", and most popular games have modding communities (except in cases where the developers didn't design a moddable architecture--personally I avoid those as much as possible!) The problem I think you're going to run into with those (if there are even official forums anymore) is that they're pretty old, so any modders have likely moved on to more modern games. And no, you don't typically need to be a programmer to be a "modder", except in some rare cases where a game gives you some powerful scripting language or such. Mount & Blade for example allows you to mod much of the game via Python. (Btw most of us here are probably developers, not modders.)