Re-reading that wiki http://wiki.dreamhost.com/Move_Domain_Between_Accounts again, it definitely implies moving a domain from one DH account to another involves significant downtime! Steps quoted there in "Moving a Domain to an existing account" are basically to:
(1) Backup old server
(2) Delete domain from old account
(3) Initiate a domain transfer to new DH account
(4) Once domain is transferred, upload all the files to the new DH account's server.
There must surely be a way to do this without any real downtime, i.e. by serving the site from old and new servers during the transition, then deleting the old server a couple days later when all users should be being directed to the new server. The site files (200GB) are already in place on the new server, ready to go live. But I don't know how to complete this transfer? Any help welcome, thank you.