I'm afraid you will have to swap all textures of all 3D objects with a manual script, I'm not aware of any easy engine switch to change which textures get loaded at runtime for now.
It may be just a script running on every instanced scene which replaces materials or textures in those materials by their lower res counterpart. The problem is, that won't prevent the initial high-res texture from loading first.
You may want to ask for this in the issue tracker (i.e a "quality setting" option to make the engine switch between materials). But doing so will often involve scripting on your end regardless.