You have a misunderstanding of textures and shaders. Shaders are what are used to position and shade an object (Vertex/Pixel shaders) and numerous other things. Textures are used within shaders to give the objects specific looks, u can’t replace a shader with a texture.
But to implement a system like u want. Can be done in Blueprints. Create a fancy material and an unlit material, then keep track of whatever metrics u want in a Game instance class. When a threshold has been reached (ie. Num objects on screen, distance, etc.) swap out the fancy material for the unlit one. But again you will still be bound to whatever version of API you packaged for ie. D3D11, 10, or OGL.