Last weekend I moved from Redhat 9 and Fedora Core 2, as well as from one physical locatiopn and IP address to another.
We did the move on the weekend because we do not normally change content files on Saturday and Sunday.
The move went easily and the folks at DA were really responsive in making the license changes to make this happen.
In my case I needed to back up directories which were outside the control of DA and move them using sftp to the new server. Once I had done that, I used the backup tools contained in DA and transferred them to the new server.
Once I had reinstalled DA on the new machine, I refreshed the installtion from the backup and had revcovered all my databases, email accounts, email, and all the files in the /domains directory for each user.
I did not need to handle a large number of users because we do not use the resale features of DA -- instead we use DA to administer our company's mail and web server environment.
However, I am, confident the process will go well for you -- even if it involves "x" times more user accounts.