g1smd - 1:56 pm on Apr 3, 2012 (gmt 0)
Change the CMS so that it links to the friendly URLs.
Redirect requests for the old URLs to the new URLs. This will retain traffic following old links and preserve much of the pagerank.
Since you're talking about Joomla, look for specific add-ons that have this capability.
Both changing the links on the pages and redirecting requests for the old URLs to the new URLs are crucial. You must do BOTH of these things.