Generating thumbnails

I’m switching my site over from a theme that does not use thumbnails to one that does. So for the most recent 15-20 posts, I’m trying to go back and generate the thumbnails. (I’ve already got the appropriate functions in my functions.php file.)

Problem is, WordPress refuses to generate thumbnails for those posts. It works fine if I upload an image and immediately click “Use as featured image”, but if I take an old image – even from a couple of hours earlier – and try to convert it into my chosen size of thumbnails, it simply will not generate new thumbnails.

Is there any way around this except (1) re-uploaded images all over again, or (2) using the Regenerate Thumbnails plugin, which has only one option: to regenerate thumbnails for every single one of the images that I’ve ever used in a post on my website (i.e. thousands)?