I know that Christopher Tolkien commented in the Book of Lost Tales, Part 1 that the reason the early version of the Ainulindale looks so much like the version in The Silmarillion is that
In this case only and in contrast to the development of the rest of the mythology there is a direct tradition, manuscript to manuscript, from the earliest draft to the final version.
Having only read parts of the other "History of Middle-Earth" volumes, I'm not sure whether we're ever given glimpses of any of those other manuscripts. Is that the case?