How merge categories?

[merefield (Robert)]

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Bulk move many topics from one category to another

Is there a query to bulk move many topics from one category to another? I want to consolidate many categories into one category and would like to make the move quickly and all at once, and also not bump the topics. Bonus: also want to tag topics first with a category-equivalent tag before moving them. Grateful for any queries or recipes. Many thanks! I’d even write it up as a howto topic if it’s good.