Retcons (i)
Of course, some of those "earlier" stories may be written later:
Heinlein's "Requiem" described old Harriman dying on the Moon, then his "The Man Who Sold The Moon" described how Harriman had put mankind on the Moon;
Heinlein described the heroic death of Space Patrolman John Dahlquist after he had shown the Patrol honoring this hero;
James Blish wrote a novel about the young Adolph Haertel after he had described the older Haertel conferring with the pilot who had successfully test flown the Haertel overdrive;
Anderson wrote a series of stories about Dominic Flandry, then the "Young Flandry" trilogy about the start of Flandry's career;
the story introducing Adzel as a student on Earth was written after the "later" story about Adzel joining the trader team - in fact the introductory story expands on a short dialogue in the "later" story;
and there are other examples.
Retcons (ii)
Also, a later work can present new information about earlier events. The peak of the pyramid can shine a spotlight on its base. For example, Methuselah's Children reveals that:
the Howard Families, bred for longevity, had existed secretly since the late nineteenth century;
thus, they had existed throughout the entire Future History to date;
in fact, one of their number had lived since the early twentieth century;
Andrew Libby was a Howard.
The History would have been better constructed if the reader had been able to recognize an earlier spear-carrying character as an alias of Lazarus Long.
Poul Anderson's The Game Of Empire reveals:
the existence of the Dakotian and Zacharian communities;
an old Merseian conspiracy involving Aycharaych, at last coming to fruition.
A future history is a work in progress. The author can always add and reassess. Larry Niven shows us that we really knew nothing about the Puppeteer Fleet of Worlds or the Ringworld.
