You can take advantage of Runtime Data Layers to hot swap between 2 sets of Props/NPCs/etc. This exactly what the Ancient Game demo is doing when switch to the “dark world”. If you want the transition to be as quick as possible, you can preload the 2nd set of data by setting the runtime state of your Data Layer to “Loaded”. When you are ready to swap, change Data Layer 1 from Activated to Loaded and Data Layer 2 from Loaded to Activated.
As for changing the landscape material at runtime, I know it’s possible but I’ll let my colleague answer that.