I partly agree with what you've said, above, in as much as decent impressions go a very long way to minimising the likelihood of seal-breakage, but it doesn't take into account that some people's ear anatomy varies more than average, with varying positions in jaw position, etc.
John Moulton acknowledges that there are wide differences in ear anatomy, and it is therefore to be expected that some people will experience more change in the shape (perhaps also size) of their ear canal when lying down (jaw can shift backwards when lying down) than other people experience when lying down, by comparison with the shape of the ear canals when sitting or standing.
Obviously, the tighter the CIEM fit, generally, then the less likelihood of the seal breaking, regardless of one's unique anatomy, but there may still be some people for whom the change in ear canal shape may be so great that it breaks the seal, when lying down, even with generally-tight-fitting CIEMs.
Having said all that, if LoryWiv's CIEMs are not uncomfortably tight during sitting/standing, then I agree that it might be worth trying a refit, but in that circumstance, I'd personally be tempted to cover my bases and get fresh ear impressions done. Not essential to get fresh impressions, but might increase the chances of success. And, as ever, the oft-mentioned thread about impressions is a worthwhile read.
Thank you all for the helpful comments. I will call 64Audio and ask for a refit, and defer to them if fresh impressions are really necessary (I felt pretty confident in the audiologist...she re-did them several times until she was certain they were right.) The sound is such bliss when well-sealed that while I hate to part with them I am prepared to do so if 64Audio agrees to the re-fit. Will report back in case anyone else is having similar issues.