Changing thumbnail sizes in existing posts

Hiya.

I’ve tried to change the default thumbnail sizes in Settings -> Media and used Regenerate Thumbnails to view the effects only to have useless new copies of my images with new thumbnail size AVAILABLE but what I’d like is to batch hundreds of posts to achieve thumbnails twice the size they are now. EDIT: To clarify: I want all my images in all my posts double in size as they are viewed within blog posts. The images files themselves do not need to be resized.

Is there a way to do this or is this just escaping the truth that I have to manually go over each and every post…? Say it ain’t so…

Thanks,
– Jaakko Pöntinen