Custom taxonomy bulk edit?

I’m trying to convert an old category to a custom taxonomy – parent category is car manufacturer and custom taxonomy is “manufacturer” (instead of “category” with the actual manufacturer (example: Audi)) as the value.

I want to convert all posts with the category of the manufacturer to the new custom taxonomy but I can’t seem to figure out how to do it. The new custom tax does not show up in the bulk edit functionality so I would have to go back through all of the thousands of posts to update this.

I’m sure there’s a better way to do this but I can’t seem to figure it out.

I would like to filter posts by the old category then add the custom taxonomy to all those posts.

Is there a way to do that?

Thank you