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I can not understand the difficulty of the web designer. Also do not understand why he speaks in the DNS migration if there are two different domains. Well, what he has to do is an exact copy of the site for the new domain in a new host plan. After the copy being hosted in the new domain, it does a 301 redirecting the old domain to new. So all pages will redirect to the new, including the intern will be right and you will not lose rankings.

Shopping comparison(SC) sites work of a product data feed which is specific to every site.  So you've got two options 1. Create a data feed for every SC site you want to use or 2. Use a third party such as www.godatafeed.com who will take one feed from you and create the multiple specific feeds Some SC sites have implemented a bidding system others use a fixed cost per click.  If you go for the first option you can use Analytics software to optimise your feed (optimise titles/descriptions to increase CTR, remove items which have high Cost/Conversion).  If you go for the second option, they will most likely optimise feeds for you.