We have currently around 40 websites and 80+ domains/sub-domains in a shared 1&1 hosting package, and we just acquired a managed dedicated server with 1&1 as well.

Now it's time to start transferring everything over to the new server. Transferring just the websites and databases wouldn't be a problem, it would take time but it's pretty straight forward.

The problem comes when transferring the domains, let me explain why. Many of the websites we have are accessible via sub-domains of a parent domain. Ideally, we would like to transfer the sites one by one, in order to check for each one that everything works fine in the new server. However, since we also need to transfer the domain so it's managed in the new server, once we do that means that all the websites using that domain need to be already in the new server before transferring that domain, thus not allowing the "one by one" philosophy.

Another issue is the downtime when transferring the domain, from the moment it stops working in the hosting package and becomes active in the new server. I believe there's nothing we can do here.

So my question is if there's any way we can do the "one by one" transferring of the websites (and their corresponding sub-domains) in the circumstances described above.

One idea I had would be:
1. Let's say we have website A, which is accessible using subdomain.mydomain.com (and there are many other websites accessible via other sub-domains of mydomain.com)
2. Transfer the files of website A to the new server
3. Point a test domain in the new server to the website A's folder (the new server comes with a "test" domain)
4. Test if website A works with that "test" domain
5. In the old hosting, somehow point the real sub-domain (subdomain.mydomain.com) to the new location of website A, in a way that user always see the same URL as always 6. Repeat 2-5 for every website belonging to the same domain 7. Once all are working in the new server, do the actual transfer of the domain to the new server, and then re-create all the sub-domains and point them to their corresponding website

That way, users wouldn't notice that there's been a change (except for a small down time of the websites when doing the domain transfer).

The part I'm not sure about is point 5 of the above. Is there any way to do that? I mean do it in a way that users see the original domain all the time in their browser, even for internal pages (so not only for the "home page", which would be sub-domain.mydomain.com, but also for example for the contact page, which would be sub-domain.mydomain.com/contact.php).

Is there any way to do this? Or are we SOL and we're going to have to transfer all at the same time?