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Some good comments here, and I'll have to come in somewhere in the middle. I think Vahe is right that there can be meaningful benefits, both for SEOs and visitors. It's also true, though, that a site-wide URL change can carry risks. Solid planning and well-implemented 301s can mitigate most of that risk, though. If it were only to get keywords in the URL and the site is ranking well, I'd probably hesitate. Since these dynamic URLs are creating duplicates, though, I think it's a different situation. Those duplicates could create very real risk to your rankings. If the URL change can solve both problem, I'd be much more inclined to do it. There are other ways to deal with the duplicates - the canonical tag is probably a good bet here (although I'm not sure how tough it is to implement in Joomla). Blocking duplicate-causing parameters in Google and Bing Webmaster Tools is another option. For example, you could block "Itemid" if it had no unique value (I'm not clear on that from the example).