Merge categories

I'm currently trying to consolidate the number of categories we have in our archival section by merging categories together. However, there's no easy way to do so. All I can do is filter on one category, select all topics, and bulk move them to the other category.

Now, this is fine if the category is not that largen. But I have several categories with hundreds of topics. And the most I can show on one page, and thus, move, is 50… 

Please create a merge function, or a true ‘select all’ function so I can move all of it at once.