Changing the compression format of multiple textures - unity3d

Question

I'm using the following method to compress a bunch of textures:
public void OnPostprocessTexture (Texture2D t) {
EditorUtility.CompressTexture(t, TextureFormat.DXT5, 2);
}
The idea is to try to compress the texture when importing it. I have a project with many textures which are not using an optimal format.
The problem is that these changes are not saved anywhere, if you check the editor you'll see that the format remains the same. I can leave the script there and reimport everything in the build server, but I'd like a way to save these changes.
The only way I can think of this is to create another texture using the format I want and copying/replace the texture. Is there a better way to do this?
Edit
Doing some more tests, I noticed something strange: EditorUtility.CompressTexture is somehow compressing NPOT textures. This is before running the script:
And this is after running EditorUtility.CompressTexture:
How does this work?
While Sergey's answer helps me change the format, it won't compress NPOT textures and I really need to save these bytes.

Answer 1

You have 2 problems here.
Problem 1
You are trying to do it inside OnPostprocessTexture method, which is too late. You need to do it inside OnPreprocessTexture method instead.
Problem 2
EditorUtility.CompressTexture compresses a texture object itself (an object that resides in RAM), not a corresponding asset (an object that resides on disk).
The correct way of doing what you want to do is using TextureImporter.textureFormat.
Solution
Here is a working example:
public void OnPreprocessTexture()
{
TextureImporter textureImporter = assetImporter as TextureImporter;
textureImporter.textureFormat = TextureImporterFormat.DXT5;
}
Answer to comment
Another detail: I don't agree that CompressTexture creates an object in RAM instead of the disk, I think it creates a file inside the Library folder (that at some point is loaded into the RAM).
No, it doesn't create anything inside Library folder. It's quite easy to check:
Start with minimal Unity3D project: only one texture and only one editor script (a descendant of AssetPostprocessor with your OnPostprocessTexture method.
Open Library folder and write down number of files and their size.
Reimport the texture from Unity editor (it will result in execution of your OnPostprocessTexture method).
Open Library folder and see that no files were added and total size of files remains the same (at least this is how it works on my machine).
Answer to edited question
Doing some more tests, I noticed something strange: EditorUtility.CompressTexture is somehow compressing NPOT textures.
If I try to reproduce it, Unity outputs an error line to console: "!hasError".
Despite that Unity editor says that the texture is compressed with DXT5 after such a manipulations, the actual texture format is D3DFMT_A8R8G8B8 when you check it in a plugin.
One thing to note here. Actually, DXT compression doesn't require textures to be of power of two size. It requires them to be of size of multiple of 4. Hope this helps :)