User friendly urls will improve you site from the SEO point of view. If your site is public and present in search engines will be benefited with this change.
I have to disagree with Sunny in relation with old urls. It's not true that users won't be able to access to old urls, normally, you can create redirect rules to send user hitting the previous format to the new one.
So reasons I would evaluate are:
- How important is to improve the site from SEO point if view
- If old urls can be translated to new url format through redirect rules and the importance to this.