When you switch domain names to a new server it is often useful to make sure that the new server is ready for the domain before you do DNS updates.
Normally when you go to a webpage your computer will lookup the IP address of a domain name from your internet providers (ISP) name servers.
You can override this by fixing the IP address for a specific domain in what is called your local "hosts" file. As ever depending if you are on a MAC or a PC there are different ways of doing this.
REMEMBER: Whenever you've finished your testing to remove hosts entries that added for your testing otherwise you may not be looking at the correct server for a URL.