Categories mess after import from WP

This is about

After I imported the two XML files from my WP blog, I wrongly assumed that deleting categories in the MB interface would also remove those from the posts. Alas, those remained, and I no longer have access to the categories per post (how they were tagged in WP).

Is there a way to get access to the per-post level tags, since I can’t see anything I could use in the MB Posts edit tab?

Right now, I have to re-upload the text of old posts as new posts, change the date, and delete the old post. This is very tedious and error-prone.