If I'm understanding correctly -- if you're going from one server to another, with the same domain name (regardless of subdomain), you'll have to wait until the domain propagates thru the Web -- about a day. That is, if you've notified your domain registrar about the new nameserver of your new web host.
Until then, you'd probably need to access the new CP via its IP address...
(If I'm misunderstanding you, then sorry...)
