Thank you, Elle and Andrew, the account credit mechanism worked exactly as you explained it would. But the process is weird. The screen which comes up when you press on "Close Account" says:
Phew. Does that give any indication whatsoever that the system is about to apply the pro-rated unused amount back on to your account as credit? No it does not! It says it is going to blow your account to smithereens, leaving you only a measly panel where you can manage and if necessary pay for your domain registrations.
Well anyway, you then have to tick a box that says "Yeah yeah, I know. I'd still like to cancel" which does finally take you to a panel where it says it is going to credit your account with the unused amount.
So you go ahead with the heart-rending goodbyes (at least, the Dreamhost panels attempt to give an impression of being heart-rent)
(and, by the way, the panel completely ignored my explanations that I am not leaving, I am simply hosting with another host while Dreamhost tackles some instabilities)
and then you get an email saying:
Again, no mention that you still have an actual account at Dreamhost, let alone an account in credit.
If you then log back in to your account, you find that (a) you still have an account and (b) it has been credited with anything up to a couple of hundred dollars. You may not be quite confident that this money is real. At this point you can google for "show me some 4-letter .com domain names that have come on to the open market after yet another cybersquatter has thrown in the towel" and test your credit with an impulse buy.