You’ve made the decision to change your domain name, and now you need to take the steps necessary to make the change without hurting your site’s rankings.
Back up your existing website, including database (if your site has one).
Register the name and set up hosting for the new domain.
Move one directory or subdomain from your existing site to the new site.
Set up 301 redirects to tell Google that the pages have been permanently moved.
Check Google to see if the pages you have moved are showing up in the index at their new address.
If it all looks good, move the rest of the site over and be sure to set up 301 redirects for each page. You don’t want to lose any backlinks or site authority your website had at the previous domain.
Don’t set up a 301 redirect where all old pages point to your new home page. Take the time and trouble to redirect each page individually. If you’re changing your site as well as moving it, and there’s no longer a 1 to 1 match for each page, then redirect the old page to a page on the new site that has similar information and will give the users a good experience.
If you are rebranding, or completely changing your site, you might want to move the old site first, do all your troubleshooting, and then set up the new or rebranded site at the new location. You’ll still need to do 301 redirects. But this way, you’re controlling the number of changes at any one time and limiting the possible points of failure.
With 301 redirects, the backlinks to your site will continue to work, but if possible, you should notify the site owners who link to you and inform them of the change, giving them the new URL to link to. Some will edit their link to you and some won’t, but it’s a good idea to try.
Don’t abandon your old domain name for at least 6 months. There will always be stray email and links and other issues that may crop up and you will want to still own the old domain so you can tie those all up.
Be sure to set up Webmaster Tools properly, Web Analytics, verify ownership, create a new sitemap, etc. Keep both the old and the new domains in your Webmaster Tools account. That way you can check for crawl errors to make sure all of your 301 redirects are working properly.