I couldn't really test site
Depending on network configuration you may be able to override the site by placing an entry in a local hosts file on your desktop to point to new ip address instead of old.
If using linux use /etc/hosts and set "order hosts,bind" in /etc/host.conf.
If using windows place an entry in c:\windows\system32\etc\drivers\hosts"
That should allow you to point to new site and do some testing.
move established setups into it
one for the developers? I think there may already be a request in there for some sort of migration module from, eg. a cpanel backup. Have a look in the ispconfig roadmap or bugtracker see if there is a feature request - if not why not request it?