Most clients have their login credentials for the company where their domain name is registered. (For example, Network Solutions, Hover, GoDaddy, etc). It's important that we're able to get to this information at some point in the project so that we can move your DNS (domain name service) to point to the new site. If your current website host also controls your domain name registration, you'll want to make sure to get login credentials from them well in advance of the date you want your new site to go live.
Typically there is no downtime during this whole process. In most cases it's a very smooth transition, and the only "blip" is the time it takes for the new site to respond at your web address versus the old one, while the new DNS figures out where to find your site. There can be an up to 48 hour delay in when the new site shows, depending upon various factors, but the old sites will continue to display during that time.