I like that idea!
This isn’t quite what you are asking for, but you can speed up the process of moving a category’s topics to a new category by bulk selecting the topics from the category’s topic list:
Click the “Select All” button that appears, then click the topic list’s admin wrench. Select the “Set Category” option from the bulk actions modal:
For sites on our hosting, if you have a huge number of topics that need to be moved from a category, get in touch with our support team. We can give you a hand moving the topics out of the category.