Raief - 11:59 am on Sep 14, 2011 (gmt 0)
If you are using WordPress there is a plugin that redirects your old URL link to your new one automatically. I used it on one site and it worked, but before I implemented it I waited too long and I lost a lot of backlinks. If you do it quickly your site wont suffer but if you are like I am and wait a long time it could suffer some.
The plugin is called Permalinks Migration. But like I said it needs to be done quickly to get the full benefit. It sounds like it might help some, but you may have allowed too much time to pass for all of the links to be switched out, unless you had a redirect for the old pages.