"How the hell did you manage that?" you might ask.
Well, I managed to do it because the default selection in XF when deleting forums is .... DELETE children!!!
I realise that now, but it seems to be quite a dangerous default.
I'm therefore proposing a simple change to XF to swap the default to ... "Attach child nodes to another parent".
This will save anyone else accidentally removing a whole section of their forum as I did.
And yes, before anyone jumps in and tells me I should have looked ... I agree ... but it wouldn't have been so easy a mistake to make if the default was NOT to delete everything underneath the cat/forum I was removing.