@hek I used redis-cli to remove everything which was not a category, a topic, a post, a user. I had less than 20 entries for each of those things so it wasn't too hard to do by hand, of course you could automate this using some script.
Then I installed the new NodeBB normally, using that cleaned Redis DB as this instance DB. It just didn't add default categories as some where already there, but it worked. I was able to login and all posts were there.
Of course you lose everything else, configuration, plugins, etc. But I think it's better. NodeBB is still a bit early to modify it too much. Plugins just started being versioned. The only thing really worth investing time in is theming, as long as you mainly do CSS and modify few templates.
I did that on a test separate instance of course (different redis instance on different port + nodebb instance clone running on different port).