Answer:

This depends on how your sites are hosted. If it involves a change of IP number then the process is a bit involved, and depends on who controls the DNS (Domain Name Server) for your domain.

For every domain there is an 'authoritative' DNS that tells the rest of the world what IP number your domain equates to. This is cached by name servers all over the world for a time specified by the TTL (Time To Live) of the A record. Typically the TTL is set to something like 24 hours.

If we just change the IP number of your domain and it has a TTL of 24 hours, then people all over the world will still potentially see your old IP number until their local name server stops caching it. For various reasons this could take up to 3xTTL = 3 days. That means people could still be seeing your old website in their browser for up to 3 days. Not good.

To effect a fast changeover, we need to reduce the TTL of the A record at least 3xTTL in advance to something much more snappy, such as 5 minutes. Now we can effect the changeover in just 15 minutes. We can raise the TTL afterwards back up to 1 day once the changeover has been made. Reducing the TTL like this causes extra traffic for the DNS server, so how low we can set it depends on how much traffic your site generates.

It should also be pointed out that a lower TTL makes it much easier to switch the IP number back again if for any reason you discover a problem with the new site.

Proxying

The new IP number will require proxying, so as to let our servers know what to do with requests to that IP number. I am afraid that we do not permit clients to organise their own proxying, no matter how well-intentioned that may be. We reserve the right to change the IP numbers of our servers as needs arise, and cannot arrange for clients to rearrange their proxying before we can do so.

Recommended practice these days is to make sure that both http://<domainname> and http://www.<domainname> will resolve correctly in a browser, so that visitors don't have to remember which one works, and don't get confused by strange error messages when they get it wrong. For this to work, there must be separate A records (or CNAMES) for all combinations.

Further, for the benefit of search engines such as Google, it is recommended to perform a 301 (Browser Redirect) from one domain to the other. We can set that up on our proxy, all that we ask is that you tell us which one you would like redirected to the other. This could apply to all your other domains as well, such as .org, .net, .co.uk, etc but that is more of a marketing question. Certainly Google is happiest if one page corresponds to just one URL, and all other combinations get 301'ed.