I don't think this will ever be implemented. As stated before, one could create a new account with the proper domainname, and copy files over. Option B is to do an addon domain pointing to the root dir
The issue is, that everything in the system depends on that domain name. 2 mail programs would have to be updated as well, and to keep the old mail, a major tweak would have to be done. Don't count on it ever happening. JS