Yeah, the "referral" is what created he problem. Note the "red" warning text in the wiki article:
Maybe, unless you are willing to reverse the referral award (tech support can do this for you), or wait for one year before that domain can be added to another account where a referral or promo code was entered.
I know another developer who just went through this with three domains that needed to be moved to two different accounts within the last two weeks. She thought she might be able to get an "exception" to the "no referral or promo" codes on accounts receiving previously DH hosted domains because the end result was the turning of one account into three, tripling DH's revenue for hosting the "same sites" (she had no intention of canceling her own account, as she has been at DreamHost for years, and still hosts many clients' sites on her own account).
She wrote to support, clarified her intentions, and asked for an exception to the rule. Tech support responded after a delay, stating that they had to check to see if an exception could be made, and said that they were sorry but they would not be able to make an exception to the rule.
Her response was to ask them to reverse the referral (it made no difference to her client, since no promo code was involved) so that she could proceed with the deletion of the domains from her account and the addition of the domains to the two clients' accounts. It was her feeling that her client was still best served by staying with DreamHost, and that was her principle motivating factor.
While a potential referral award would have been nice, she understood that the cheap bastards that are always trying to game the system for continued hosting at promo prices kinda queered the deal for the rest of us by making such a rule necessary. Once the referral was reversed, support notified her that all was "ready to go, and she moved two domains to one account, and a third domain to another, with no further issues.
Yeah, if the domain is not expired it's a moot point as far as the hosting goes - you can effectively "change" the ownership of the domain by changing the contact info for the registrant in the WHOIS record. None of that is really relevant to the hosting anyway if the domain is not registered at DreamHost. If it was, they would have to transfer the domain registration between the accounts, but that is not relevant in this case.
It's complicated because of the referral/awards situation - at some level, I understand it while on another I think cases like this actually hurt DreamHost's revenue if it causes a customer to host somewhere else. That said, if a domain has already been hosted at DreamHost, it's not like the hosting is an "unknown quantity" that should need an incentive to decide to host here (which is what the rewards/promo codes were (apparently) primarily designed to accomplish ... it's a complicated subject, and I see both the developers' and DreamHost's positions as being reasonable.
If you want to transfer it to another DH account, then the reversal of the referral is the way to go - you can always ask for an exception, but this last incident I am aware of makes me think you are unlike to succeed with that.
Of course, you can always move the client to another host also. If ASAP is important, I'd go with the referral reversal - I helped my developer friend move those three domains between accounts and, with proper planning and staging, there was a total of less than 5 minutes of "DNS strangeness" when we dropped the domains from one account and added it to another. You may not be able to achieve such a "clean" transition, with so little downtime , if you change hosts. Plus, your ex-client will have to find another host, and DreamHost will still end up comparing favorably on features and price, so why move?
Good luck whatever you decide to do.