Editing lightmaps through code

Hello!

I'm currently working on a game with a cel-shaded look. However baking out lights for our environments results in a standard lightmodel being baked, which clashes with the visual style. I came up with the idea to hook a little editor script into the lightmapper that takes the baked lightmaps and re-clamps their grayscale values to mimick the new lightmodel.

However I seem to run into trouble when accessing the texture's pixels. Normally I'd use Texture2D.GetPixels and SetPixels, but since Unity-generated lightmaps are compressed into a DXT5 format I cannot use this. I've managed to eventually get the data by creating a copy in-memory and editing that, but I can't figure out how to assign it again. Here's the relevant code:

     Texture2D lightmap = AssetDatabase.LoadAssetAtPath<Texture2D>(p);
     Texture2D texcopy = new Texture2D(lightmap.width, lightmap.height, TextureFormat.RGB24, false);
     texcopy.LoadImage(lightmap.GetRawTextureData());
     
                 Color[] pixels = texcopy.GetPixels();
                 for (int i = 0; i < pixels.Length; ++i)
                 {
                     //Change all the pixels here
                 }
     texcopy.SetPixels(pixels);
     texcopy.Apply();
                                   //Here it seems to break?
     lightmap.LoadImage(texcopy.GetRawTextureData());
     EditorUtility.CompressTexture(lightmap, TextureFormat.DXT1, TextureCompressionQuality.Best);
     lightmap.Apply();
     DestroyImmediate(texcopy); //This might not be needed, but cleanup to be sure

Anyone knows how to properly assign this, or perhaps knows a way to edit the texture's pixels without needing to make a copy?