I think I'm right in saying once an event is given a category, there's no way to then change it's category.
I'm having people post events in the wrong category and hoped as admin I could move them....
What would help also is if on the front-end 'Select Category' page the description of the category (or at least the first x characters of it) could show underneath it's title.... Admin provides a description when setting the category up so I think it would be nice and helpful to show that front-end when a user has to pick a category.