COMIC LEGEND: Superboy and Supergirl were going to depart the Legion of Super-Heroes years before they eventually did, but DC backed out mid-story.
1966's Adventure Comics #350 was a bold issue for fans of the feature. Writer E. Nelson Bridwell (also assistant to Mort Weisinger, editor of the title at the time) featured Superboy and Supergirl both forced to resign from the Legion due to a cloud of green Kryptonite in the future.
The issue and the next brought back a lot of former Legionnaires and it definitely seemed as though the book was being set up for a brand-new, Superboy/Supergirl-less, status quo.
Which is really pretty impressive, as the characters had only gotten their regular feature 50 issues earlier.
However, in the very next issue, the Kryptonite cloud that had "forever barred" Superboy and Supergirl from visiting the Legion (and which Element Lad could not use his powers to change) was suddenly turned blue by Substitute Legionnaire the Color Kid!! Everything was now okay (blue kryptonite is harmful to Bizarros but is fine for Kryptonians)!!!
Sounds like a pretty abrupt turn of events, eh?
As it turns out, the story was changed IN THE MIDDLE OF THE STORY!
Bridwell recalled the event in the 1970s in the pages of The Legion Outpost (the famed Legion fanzine)...
"[Adventure Comics No. 351] fixed things up for a lot of the Legionnaires... Mort [Weisinger] and I sort of plotted as we went. In Part 1 [Adventure Comics No. 350], we wrote Superboy and Supergirl out of the Legion and intended to leave them out... Then, when the time came for a sequel, we had orders from the bosses to get Superboy back. Changing the color of the kryptonite cloud surrounding Earth from green to blue was a bit far out, but it was the only solution we could come up with, since we'd established that Element Lad couldn't do the job."
Obviously, years later, the Legion DID move past having to have Superboy around, but it's interesting to note that Weisinger wanted to try it many years before it actually went down (and, of course, it's interesting to note that DC quickly squelched the idea)!
Thanks to the great Legion Chronology website for the information (and, of course, thanks to the Legion Outpost and Bridwell)