While taking up an existing project, I realized that most of the existing textures are at 2048 x 2048 resolution. This was taking up a lot of memory since the project has a couple hundred textures and a lot of materials that use these textures. I wish to reduce the texture sizes but I am unsure of if the materials using these textures will need me to add the textures back into them individually, which would be the last thing I want to do.
No, you don't have to add textures back to the material associated with the textures for the future. As long as there's a texture in the material already then you can adjust the texture resolutions. You can even adjust them accordingly per build platform. You can select the texture/image file in Unity hierarchy and do a few things like set platform-specific overrides which will change the texture resolution automatically when you switch platforms fyi.