From time to time companies need to change their website url for different reasons. Here we will cover changing domain names, and what you should do with your old domain (and content) and what is in store for you with your brand new url. Also, you should only change your domain name if it is absolutely necessary!
First off anytime you are dealing with changing domain names, you will still want to keep your old url registered, as you will be setting up a 301 permanent redirect to your new url. 301 redirecting your old your to your new one will help with transferring traffic flow to your new url and with passing on link juice to the new site. Now, it is totally safe to move the content that was on your old site to your new website. You will not need to worry about any duplicate content issues as long as your old site is properly redirected using the 301 redirect method.
Generally, with changing domain names, you are starting over from scratch with your link building as the new site has no incoming links. All the links that use to link to your old site, you will need to contact all those sites and see if they will update your link to the new url. The link juice from your old domain will pass on to your new site, but with a 301 redirect, studies have shown that the full amount of the link juice does not pass on. You will want to gather a complete list of the websites that link to your old url, and contact them to update to your new url. it will take some time to get the site back off the ground again, but this is a natural phase that the majority of sites have to go through.
With your new url, you will want to check to see if this was previously owned by anyone else. If so, then you will want to do some testing on the new url to make sure that the new url will rank, and that there are no problems. If you are starting with a brand new clean url that has never been registered before, then there should not be any issues. But testing is a good route to go to verify everything.
When moving multiple websites into one domain, it would be best to start with the site that is receiving the less amount of traffic for the first part of the move, then move on to the next biggest site and so on (traffic wise).
If you are also planning on changing the website template layout and content, etc, testing this out will be essential. Anytime you change the content, layout or navigation structure of a website, changes in rankings are very likely to happen, so you can try testing out portions of the site at a time to see how the results are showing, and see if there are any alterations needed.
SO, make sure you have an XML sitemap of your old domain, and that it is submitted to Google and Bing.
Register both the old and new domain with Google Webmaster Tools. Google WMT will be very helpful with domain switching, letting them know what is going on.
With the 301 redirects. Aim for a one to one redirect, that is, for each page on your old site, have the www.notawebsite.com/services.html page redirect to www.new2uwebsite.com/services.html, and so on for the rest of the site.