Firstly, why would you want to do that?
Earlier this year I tried (and failed) to do this when a client of mine at an estate agents called Redmove rang me one Friday and asked me to help as his previous site had gone down. He asked me whether his new site was ready to go and should he just switch over rather than trying to contact his previous developer to get his previous site fixed.
Where there is an existing website I always build the new site and publish it to a test site of mine so that the client can see (and approve) it. There is generally a bit of back and forth until the client is happy with what they are about to get (which is fine). I would rather the client is happy.
Moving from the test site to a live site has to done carefully with lots of attention to the detail. Normally when publishing to a live site I can check it straight away because I can see it in a browser and go through it with a fine tooth comb to spot anything wrong and fix it immediately.
In the case of moving an existing domain name to a different server this is not possible. Typing the domain name into a browser window will always take you to the old site until such time as you initiate a change to the name servers to point to the new site. This takes hours if not days to occur. Wouldn't it be great if you could 'see' the new site on its new hosting and check it out before committing to re-pointing the domain to its new hosting. I tried to do this for Redmove and I couldn't get it to work for me so I had to fly blind and trust that my own implementation plan was up to the job. As it happens, everything worked fine and the client was happy.
I am currently working on a site for MAI Design in Boston MA and they have kindly pointed me to the following method of viewing a site by I.P. address. Warning - this requires some changes to the guts of your Mac and it requires fine attention to the detail so its not for everyone:
These instructions are provided by a web hosting company called Name Hero. Use with caution.
How To Test Or Preview Your Website Before Switching DNS
One of the most common questions we get is, How Do I Preview My Website Before Changing My Name Servers