to migrate from one server to another is very hard to do.
to migrate the web is not the problem, but how to migrate the mail-server with no downtime (ttl of dns!)
we discussed this feature many times but at this time there is no plan to implement it, because of the heaviness of this feature.
maybe you can take a look at openvz.
openvz is supported in the monitor of the SVN-Version of ISPConfig
(DO NOT USE IT AT PRODUCTION SYSTEMS!!!)
with openvz migration from one hardware to another is no problem!