A while back, a dev called Myles Johnston started to add features to ScummVM which allow the modding of old SCI-engine Sierra games with high-resolution graphics. These graphics can be either newly created assets or AI upscaled. But all you really need are the original game, the upgraded graphics pack and ScummVMX.

Here's a link to Myles' YouTube, where you can see his work in progress.

And here's my small attempt in creating an HR mod for Colonel's Bequest. The link goes to my Blogger blog.

But basically, what this means is, that it's now possible to easily, at least relatively, to upgrade old Sierra games. Myles is even adding features for making the old games fully voiced.