Its easier to do them one domain at the time vs one user at the time.
The question has been asked several times before.
Do a search and you will find the answers.
Its pretty much like this:
Create the domain on the new server.
Disable the domain on the old server.
Delete all the content from the domain folder on the new server.
Copy the content from the domain folder on the old server to the new server domain folder.
Then on the new server reload the domain. You will need to enable it on the new server and add the fqdn for the domain. Its all in the general tab for the domain.
You will of course need to handle dns, certificates, etc too.
Just do the search, you will find instructions on how to do one domain or all of the domains on the server.