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Sometimes you just have to upgrade your platform. When this happens, it's also sometimes a good time to change your URL structure - especially if your old URLs were long, ugly, full of parameters, and in general bad for click-throughs. As Ben said, if it makes sense to keep your old URL structure (and it's technically possible) this is your best bet. Barring that, moving everything via 301 redirects is not unsimilar to an entire site migration across domains, although it is simpler. Keep in mind there is some lose of "link juice" through a 301 redirect. I can't give you a precise number, but most SEOs believe it's about 80%. With this in mind, it's important to build new links after a large URL structural change, and make sure your internal link architecture is solid. One piece of advice. I would make sure to maintain 2 xml sitemaps. One containing the old URLs, and the second containing the new URLs. Submit both to Google via Webmaster Tools. This way, Google will still try to crawl the old URLs and process the 301, dropping the old page from the index faster. Without this step, Google may keep both pages in its index and ding you for duplicate content. The process may take a couple of months for Google to index all the new URLs and deindex all the old ones. When this happens, make sure to remove the old sitemap. For more tips, you might find some value in these site migration guides. http://www.seomoz.org/blog/web-site-migration-guide-tips-for-seos https://seogadget.co.uk/domain-migration/ Hope this helps. Best of luck with your SEO!