Well, I’ve set the parent category manually, so it’s ok for now, but I’d like to not have to do it again next year 
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We use our forum to discuss the tests of our software with external users. We have a new version each year, and we don’t want to mix new tests with older discussions, so we create a new category for each version. The sub-categories are mostly the same each time so I don’t want to create them by hand again and again 
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I use this tutorial to export and import the json files: Move topics from one Discourse instance to another , except that I do it for the same forum so I change the slugs and ids to avoid conflicts.