If you are going to duplicate the sites then the transfer process should be seamless. Once the sites are ready to go, you switch name servers on all the sites and over time, people (robots) can funneled to the new site.

Issues come if you have DB's but if you are just upgrading/changing servers within the same network that would be seamless too. Update the DB to the new server, change ALL the sites to reference the new DB and then do the switchover. Anyone hitting the old site while DNS propagates will still be referencing the new DB.

But what do I know, I am still using an abacus.

Arthur