How to bunch categories into one

If you take a look at this thread – http://wordpress.org/support/topic/mergerename-categories-plugin-not-working-with-27?replies=19

Michael H says this:

You know you can delete a category and have the posts that belong to that category be assigned to the Default Post Category? So the trick is to make the Default Post Category be the category you want to hold the posts of the deleted category.

It’s a little boring, but does handle renaming and merging needs.

Perhaps that could be an option for you?