Hi,
Thank you for your quick reply.
On phpbb I had a category A and a category B.
After my import on Discourse, I had a category A, a category B, then I moved all topics from A and B to a new category C, and I deleted my categories A and B.
I want to re-create my categories A and B (which is done in seconds), and get back my C topics in their original A and B categories. I can’t do this manually by selecting each topic one by one on hundred topics.
I don’t want to do this on my whole forum, it’s “just” hundreds of topics on several thousand.
Uh… 
Even by some rails command? 
Any risk removing posts from the database directly? The Discourse forum isn’t active yet, no user activity at all.
After having imported from phpbb, besides configuring stuff, no topics or replies were posted etc.
I also don’t want to do a full re-import because I spent so much time configuring stuff since my import.