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It's easy enough change the subcategory. 301 redirects will automatically be created so the value of incoming links would be retained.
Are there any downsides to doing this from a SEO (or any other) perspective?
If they do, make a note of who's linking and see if you can get them to change the urls. This is more important when you're changing domains, I feel, as in that case it sends a signal to the engines that the change is legit. Since you're keeping the same domain, it's less important, but it can't hurt.
Use Yahoo Site Explorer to check the links. It gives a much more complete list than Google does. Registering your site with Yahoo, btw, would give you a more complete list of backlinks (inlinks, they call them) than you'd get from them otherwise.
If you are moving articles from one to another there is no issue in wordpress CMS. But if you are renaming or change the name of any category or sub-category then generated URL will be new one. Then you will have to redirect OLD URL to new one. Because there is no auto process to be 301 redirect in wordpress CMS.