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The old links that are already in google need to be given a "forwarding address" (301 redirect) to the equivalent page on your new site. The user goes to the relevant page, and all is good. If there is something common between the old site and the new site in the URL (where each had /productabc/ in the URL) then you can set up a rewrite rule with a regular expression that will take care of a bunch of the URLs at once. Otherwise, you do need to rewrite them individually.