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To complicate matters they really need to change some of the site structure too. ie current
I want to trim this down or at least make the directory structure more seo friendly.
My first thoughts are to launch the new site and gradually 301 sections of the old site to the new over a period of time. I know there could be duplicate content issues but my experience says Google will just show the most relevant page and not give any penalty. But as it is such a large site, who knows...
Rebranding to a new domain can be a very sticky problem, especially if you also want to change the url structure at the same time. That part alone is a lot to chew on - see [webmasterworld.com...] (also in hot Topics).
Getting existing backlinks changed over to point directly to the new domain - and getting new backlinks as well - should be part of the plan. The rebranding alone should be one way to attract new links.