hey there!
i think you have different choices here.
instead of unloading them, you can just hide them. it might help for rendering performances but still uses memory.
overwise you can store the assets data that you need somewhere it will persist (like in game instance or savegamesystem)
you can for exemple, store the transforms of your actors before unloading the level , and then on level load, restore them in their previous location. a good method would be to use interface for that.