If you are worried about losing the ranking of your site after moving it from old domain to new domain then stop thinking over this issue anymore. Site ranking can be maintained despite the fact that you have planned to shift to a new domain. The last thing you would want to avoid is 404 error (file not found) when a user is on the search engine looking for your site. Following tips will go a long way in maintaining your visibility.
If redesigning of your site has become essential then you can plan it out in phases. In the first phase relocate your site. Once it is done, in the second phase you can redesign site. Users are subjected to one change at a time making the process much smoother and hassle free. Another advantage of doing it in two phases is that it becomes easy to solve unexpected behavior.
Use a 301 Redirect to Redirect Site Permanently
Another recommendation for you is to use a 301 redirect to redirect your site permanently. Use of 301Redirects tells the users and the search engine that you have moved. Do it smoothly first move a section or directory and then move the rest after ensuring that your redirect is working properly.
Consider Page-to-Page Redirection
Using a single redirect to forward all traffic from your previous site to the new one might help you avoid 404 errors, but it is not recommended since it can cause inconvenience to your users. Rather page-to-page redirection (1:1) seems to be a better option, otherwise you can arrange for page-to-page transmit from every page in your old site to the new one with a similar content. Although page-to-page seems like a difficult task but it will help you retain your ranking which is very much desirable.
Contact Sites Linking to You for Updating the Links
Look for both internal as well as external links on your site; if possible contact each webmaster of every site that has a link to your site asking them to update the changes. Intimate those, about the new domain of your new site, if this is not possible then ensure that every page with incoming links is updated by new links directing to your new site. See all the internal links of your old site and forward them to new domain. After completion of content transfer from old site into the new one take help of link checker such as Xenu, which can help you detect broken legacy links. In case there are absolute links in your previous sites such as (www.example.com/cooking/recipes/chocolatecake.html) instead of links that are relative like (../recipes/chocolatecake.html) then it becomes all the more important.
Retain Jurisdiction of Old Domain till All Set for New
Retain jurisdiction of your previous domain for at least 6 months to avoid confusion. Notify Google regarding your move through Address tool in webmaster tools. In order to use the alteration of Address tool in the webmaster tools you must be the authentic owner of both the old and the new site. Include your new site into your webmaster tools accounts and authenticate its ownership. Create and submit a sitemap which enlists the URLs on your new site. It’s a way of telling the Google regarding the new pages of your new site, which might otherwise go unnoticed by Google.
Use Webmaster Tools to Monitor Everything it Closely
Use your webmaster tools data to ensure that everything is working properly. It may take little while before your old site and new sites are crawled by Google. Ensure that both old and new websites are free of web-crawl errors and make sure the 301 redirects to your new site. Also look for 404 errors from affecting your new site. When a sitemap is submitted to the Google the details page will let you know how many URLs in the new site has been crawled and indexed by Google.