Consolidating Categories

Through some fluke when I moved from WordPress.com to my own server, I ended up with double categories. I didn’t have time to both with it at the time, so I just dealt with it. Now, six months later I have close to 500 posts which are spread across dozens of categories, sometimes duplicates.

It would be a massive undertaking to go post by post to change the categories. I’m wondering if anyone knows of a way to do this in a less cumbersome way. Maybe a plugin, or a trick elsewhere. I was thinking maybe something existed to map all the posts on one category to another?

Any ideas would be hugely helpful. Thanks!