Carsten Cumbrowski wrote an interesting article on Remapping URLs with data from some recent studies on the subject by real life websites.
Read the article and see why a Good SEF Component for your Joomla based website is "must have!"
And with the majority of these components the old URLs are picked up and redirected to the right page without any effort on your site.
Think about it…. changing your Joomla website to get better rankings without loosing the old links, doesn't that sound like a win-win situation!
You already know my favorite to do this sh404SEF