Simple way
Hi,
You can move domains between clients by (mis)using Migration tool. (but you'll need at least 2 servers)
Migrate the single domain to the other machine, and after that migrate it back to the first server on another client account.
I'll hope that SWSoft will create a function which will migrate domains to clients on the same server, shouldn't be a big difference. I have found a "leak" in this Migration tool, when you input a second IP (not the main IP) from your server, you are able to migrate to "yourself". Unfortunely Plesk disables all domains which are already on the server, a little tweak in the psa db could be a solution who knows ? Also when the domain is disabled, Plesk knows that it is already on the server. (Sh*t)
So try the first solution, while SWSoft (or someone) finds a way to do this by migration manager...
Regards,
Bart