It will be possible with mod yes. Once modding is finished you'll be able to change any art, sound, textures, ui and most of game logic.
There will be convenient built-in mod manager aswell so installing or updating mods can be a simple as clicking one button and then select it when starting new game (so for example you can have multiple game saves with different mods, or you can join multiplayer game that has mod you didn't have yet and it will just work).
That's one of reason development is a bit 'slow' at the moment, I need to polish scripting engine and UI before moving forward, also decided to rewrite garbage collector for reliability, also rewrote UI (old one was laggy and hardcoded instead of being scriptable).
Btw original content isn't "baked in" in any way, GLSMAC can just load it if scripts tell it to (see
https://github.com/afwbkbc/glsmac/tree/main/GLSMAC_data/default ). And mods can just have different scripts that will load different stuff (such as custom content shipped with mod)
Check this too
https://github.com/afwbkbc/glsmac-demo-mod (it's simple but demonstrates structure of mod and how it adds/removes stuff from game)