... the post-by-post feature is perfect for what I needed to do, which is fix a handful of recent posts that I carried over from wordpress.com. For some reason, the Process all posts is listing all posts and their images, but if I go check the source of those posts the images are local. No idea what's going on there, and too scared to run it after all the hoops I jumped through to get this site moved over. :)
But if you go to a single post, you'll see a box showing all the external images that can be imported. Check the box and hit update and you're golden.