As the account is IMAP. Simply remove it and add it again. Yahoo should have refused to actually complete the move and adding it again should see it in it's original place.

Thanks for replying, Matt. Unfortunately, I've already tried your suggestion on five separate occasions, each time with no luck. No matter what I do—including deleting Thunderbird and downloading it afresh—Bulk mail always returns as a subfolder of Trash. This is so despite the fact that when I open the account through the AOL site, Bulk mail is right where it's supposed to be.