Is it possible to swop the associated topic through the rails console?
I’ve had a test, and it is possible. You can update the categories.topic_id with a new one, and then move the original ‘About’ topic around like a normal topic.
My new question is, is it advisable? 
Edit: My test site has gone a bit hinky, so I’ll remove the details in case this is the culprit. The adventure continues.