How to have 2 sets of textures and make sure only one is loaded?

My game gets memory warnings and crashes on older iOS devices. (iPad 1) So I am checking the iPhoneSettings.generation to figure out what it's running on- but now what? It's easy enough to check with anything that's going to be instantiated, but for objects that already exist in the level— do I need to make 2 copies of each level and load the appropriate level; or can I Destroy one or the other of two GameObjects and its texture will go away? Does just having a reference to a prefab declared as a gameobject variable somewhere in a script mean a copy of that gameobject is loaded into memory with the level?

What's the usual way of handlind this situation?