I tried using Batch Categories 0.91 from climbtothestars.org, which is linked on one of the codex pages. Errored out on install, don’t know if that’s because it’s out of date (it was posted in 2004) or because I’m not savvy enough.
I’m guessing I might also be able to do this with a direct database hack, but no idea how to do that. Suggestions welcomed.