If you need to bulk move all topics from a category to another category you can use the rails console.
Topic.where(category_id: <from_category_id>).update_all(category_id: <to_category_id>)
Doing the move through UI, you are not able to select many topics (I think only the first 30 or so) so if you have many topics in a category you should repeat the move many times before finishing.