If anyone else comes looking to this topic in the future, the way I had to do it was to transfer the whole topic to a new one (to preserve the old content), then deleting the old topic.
I first tried this while the old topic was in a non-public category, but then it didn’t work (users would see a login screen instead of being redirected). But then I un-deleted it, moved it to a public category, and deleted it again—and it worked!
For me it was that I wanted to move content that started out as a topic on my forum last year into a blog post format (so I can easily update the date of the content for SEO reasons). The topic was doing well for certain terms, so I wanted to transfer that “SEO juice” to the updated blog post.