According to the webhoster:
So this site and all other affiliated sites to this one will be down for approximately 12 to 24 hours on Sept. 22 (GMT -5).* Your server will be hosted in modern environmentally-friendly datacenter, thanks to the hydro-electric capabilities, provided by the Niagara Falls;
* Superior connectivity, provided by fully-redundant multitiered 10GE network with excellent throughput and low latency to both Europe and the US, due to strategically beneficial location on the East Coast (New York).
Migration itself incurs up to 24 hours of downtime. Unfortunately, this is the fastest and most trouble-free way to accomplish this.
IP addresses of your server will change. You will have to update your DNS records and, possibly, DNS server settings for your domains.
Our webmaster will handle all the DNS and other technical work required. He has also stated that he might be able to have the sites up on Sept. 22 as a 'read-only' that is we won't be able to make posts, but we might be able to at least read some posts from the previous days until all sites are back up online at the end of the day or early Sept. 23 (New York time).