Moving a blog post to another category?

Hello DSQ,
I have a ton of questions as I`ve began work on 2 new sites that are a little different than what I`m used to.  I`ll start with just 1 for now.

I`m using categories as my main navigation.  Say I`ve written a post under one category and decide later to move it to another.  What are the downsides to this?  Do I need to do a 301 to the new category?  Are there other things I need to do to correct this?
I`m using wordpress.