I would recommend never ever changing a URL unless you have a very well testes redirect script. Some of my best traffic comes via old pages that I had long abandones, but never deleted. A few years later a change in search algorithms and suddenly that page ranks highly. Dont risk changing IMHO.
As do I. I also have independent third party verifiable studies to back up my claims as well. But if you don't want to debate this, that's fine.
I have to agree with Dan. With 301 redirects there is little to no risk, plus in the case of wordpress with the redirect plugin things are even safer. So considering that the risk is really low it is always better to have an optimized URL structure. If you take 2 websites with great content and thousands of "quality" incoming links 99.9% of times that one with optimized URLs will outrank the other, that is a fact. Just by having a great keyword rich URL you won't gain top positions, but considering that the Google takes into consideration URLs structure together with many other different factors, I don't see why you wouldn't want to optimize them if it is only going to take 5 minutes.
I changed the permalinks, and used the SEO platinum plugin to do the 301 re-direct. I have validated rhe pages and images and found no errors. It seems google already likesthe new structure as the number of indexed pages have increased quickly