Ages ago, i made a tool for Fallout 4 that is really just a frontend for xtexconv, to reduce the resolution of textures that are of a specified size. But there is an inherent quality loss when re-compressing a DDS image, like doing [b].jpeg -> reduce resolution by 50% -> save as .jpeg[/b]. Same gig with MP3 files.
So I was thinking. Instead of re-compressing the .dds image to fit the new dimensions, what about just removing the preceeding mipmap levels? So if a texture is 4098x4098, mipmap 1 is 2048x1024, mipmap 2 is 1024x1024, mipmap 3 is 512x512, you'd just remove mipmap 0, and mipmap 1, to make it into a 1024x1024 texture at mipmap 0, 512x512 at mipmap 1, etc.
Is that possible? If it's just that there isn't a tool to do that, and there are APIs that can accomplish that, I'd get to work on it. Just need pointed in the right direction.