yes, there is.
You can find a post about it here somewhere.
But basically its something like this.
Create new domain with an admin user on the new server.
On the new server delete all the files in the domain folder.
Stop the mail services
Copy all files and folders from the old server.
Start the new mail server.
It will convert the users to the new format.
you are also supposed to be able to use the attach domain feature but not tried that.
Find the post before you do this since my memory might have missed a step.