decrease the quality when the player isn't near to the models

I'm creating maps larger and start to have performance issues and this problem may also arise in a variety of users who will be playing my game. I would like to be that when the player is far from a 3D model (or an entire zone) then the model loses quality (texture) or it disappears and does not load as long as the player does not come close again to that model. A bit the same method that is kept here in modern video games. How can I accomplish this? there is already a script pre-filled?