Are you changing design of your website? Or moving the old site to a completely new domain? Following are the points to keep in mind from SEO point of view
1.You have to be very careful while change of address of URL to avoid 404 (File Not Found) errors when users click on links to your site on any search engine.
2.Ensure that all the urls that are in search engines index need to either redirected (301) to a new page on the newly developed site or the page with the same URL does exist.
3. In case you are moving from an old domain to a new one, then the old domain itself should permanent redirect (301) to the new domain.All the content from old domain can be copied over to new domain.
Also to accomplish smooth transfer, you can go to Google webmaster, then Configuration ->Change of Address, and then follow the instructions listed below:
1.Set up your content on your new domain.
2.Redirect content from your old site using 301 redirects.
3.Add and verify your new site to Webmaster Tools.
Then go to the Tell us the URL of your new domain section and then select your new site from Select a verified site and tthen go to Submit.