Graham,
I just learned something, triggered by your comment about "changing formats" from Topics to Social (or vice-versa). I experimented and did just that - created a Q & A forum in a weekly-formatted site, made posts as students, then changed the format to Social.
Lo and behold, the Q&A forum seemed to disappear, replaced by the Social forum (empty, of course). But... I then noticed a block called Social activities with the Q & A forum and the Social forum both listed!
I swear I've not seen this before, but it's probably been there all along (not many of our faculty use the Social format for courses). Notice, too (in the image) that you can also add resources and activities to the Social format... bizarre!
Well, not really bizarre, just surprising (at least to me).
Oh, well. I guess I couldn't help, but I did learn something.
Take care!