You can change assets on the fly, yeah. Usually with stuff like making a gun blue you'd just load another texture and apply it to the material. It really depends on what the game is designed to do. For example a game where all the lighting is baked would have issues if certain parts of the level were changed in real time because you'd need to rebake the lighting (or add some dynamic lights specifically for certain objects)

Stuff like creating a quest in real time to the extent of hand crafted quests doesn't sound like it's quite there yet but there doesn't seem to be a technical limitation there other than what AI can do and how to refine it to do that in an interesting way. You never know but it still feels a bit early considering how little has been done so far.