Jill Whalen, head of High Rankings and as good as they come on SEO, responds that changing URL’s to have keywords in url’s is of little value in search engine rankings.
…I definitely wouldn’t change any existing URLs if I were you. There’s really no benefit to having keywords in them, and there are lots of things that can go wrong in the redirection process, not to mention that it can take many months for the engines to ‘get’ that they are now the same page.
If you can keep the old URLs, but just change any new URLs to your new format, that would be the best solution. There’s such a misconception out there about using keywords in URLs that people go to great lengths to change them, only to be really, really sorry in the end as they do more harm than good, at least at first.
We get clients who want go to great efforts to get all types of domain names (url’s) and point them at their main blog url. We advise against it. Time would be better spent working at ways that have a far greater impact on search engine rankings.