Google,recently announced a new feature in Google Webmaster Tools.
Therefore,they have given some tips that will help you retain your site's ranking in Google search results.One of the tips refers to changing the domain name because of a redesign of your site.
"If you're changing your domain because of a rebrand or redesign of your site, you might want to think about doing this in two phases: first, move your site; and secondly, launch your redesign. This manages the amount of change your users see at any stage in the process, and can make the process seem smoother. Limiting the number of changes at any one time can also make it easier to troubleshoot unexpected behavior."
You can read more details about all the possibilities on their page.
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