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Assign Missing Categories
v1.2 · by Stephanie Leary
Assigns the default category to all posts that have no category after you've converted your categories to tags.
About this plugin
If you have converted categories to tags and are now left with a lot of posts that have no category assigned (that show up with ‘Uncategorized,’ unlinked, as the category under Posts), you can use this plugin to assign the default category to all those posts at once.