If you are a moderator of a forum that has threads with large attachments, you can edit the post, then at the bottom of the page where you add/edit attachements you can right click the attachment link, then save it to your PC, then browse and select that file, then hit UPDATE ATTACHMENT. it will re-upload it and create the thumbnail.
I checked...no mass update function in the admin tools.
I'll do it as I go, if you're a mod and can lend a hand that would be great.