Resize attachment images after upload

I am messing around with themes for my new photoblog and consequently am changing the size of the main image in a post to conform to new layouts. Or I would if I could figure out how.

When I upload an image with the media attachment panel, it generates a medium size and thumbnail size depending on the setting in the options. I can then insert those or even the original size into my posts, etc. How can I go back and update the medium and thumbnail sizes if I change that option later? I tried changing the options but it only affects new uploads, and I see no way to update the image size in the media library panel.

If I decide to change themes and need a wider or thinner image 6 months down the road I’d be stuck resizing all these by hand, or deleting them all and re-uploading them with new settings. There must be a better way.