Voice acting has been good enough in the past few decades that it’s usually usable without re-recording. Maybe remastering at most.
There’s also plenty of tooling to upscale textures too, even without AI. The biggest hurdle (for assets, at least) is 3D models. You can slap a subdivision modifier on them to make them higher-poly, but you’d still have to make sure the UVs didn’t get messed up (or whatever it is that they’re using these days). And also verify that nothing weird happened like new geometry hiding or showing something in-game. (Collision probably doesn’t need to change if you’re just increasing polycount.)