Dropping a model from hi-poly to low-poly is actually quite easy. If you're smart about it, you can keep the same UV unwrap and rigging (usually, you rig the higher poly one, and the engine matches it to the lower poly rig, as when you're zoomed out accurate rigging is less important). It would be easy, if a little time consuming, to take all the models already existing and make lower poly versions of them. Going from low to high, is a bit more difficult however.