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Thanks Jonathan, I found this as a guideline for 301 redirection: http://www.seomoz.org/blog/url-rewrites-and-301-redirects-how-does-it-all-work

The thing is their website also have on page SEO errors such as duplicate page titles, duplicate content on different product pages etc. Shall we correct them all before moving to another URL structrure? Does that make any sense? Or building a correct on page SEO structure on the new CMS would be okay?

One of my customers is in online jewellery sales business and they are going to change their CMS; the problem is there is no co-relation between URL permalink structure of the current CMS and the new one. What should we do so that we don’t lose our current organic traffic (also Google rankings) coming from indexed URLs in Google due to permalink structure change ?

Typical example:

Current URL: http://www.abc.com/pirlanta-nedir,AR-3.html

The new URL is going to look like this: http://www.abc.com/tek-tas-pirlanta-ar-4.html