As I remarked a few years ago, Weisinger obviously recognized the problem with introducing Supergirl to the same Legion that Superboy belonged to; that would mean that Superman would not be surprised when Supergirl landed on Earth, since he'd known of her eventual arrival since his teenage years. So he took care of the problem, except that when the Legion became a continuing series he and his writers obviously wanted to include Kara in their adventures. So, as discussed by Commander Benson:
Weisinger’s explanation became official with the next issue, # 334 (Jul., 1965). “The Unknown Legionnaire” was one of those rare Legion adventures in which Supergirl played a large part, and it didn’t take long after the super-cousins appeared side by side that a footnote was inserted, establishing that Supergirl had implanted a post-hypnotic suggestion in Superboy’s mind so that he would forget her existence when he returned to his own time. Thus, paving the way for his total surprise as an adult when cousin Kara landed on Earth.
And so, when Weisinger reprinted this story in Action #334, the dialog was rewritten:
Note that this also gets rid of a bit of Lysenko-type science. Lightning Lad acquired his powers after a run-in with a lightning beast; there would be no reason to expect his son to inherit that genetically. Of course, there are many examples of this assumption appearing in the comics, but realistically the only parents who could give their powers to their offspring are those that have them from birth.
Some other examples of edited reprints in (or of) the Silver Age:
The second origin of the second Two-Face. The second Two-Face was Paul Sloane. In his original appearance in Batman #68, the hideous side of his face was the result of a jealous husband; when the tale was reprinted in Batman Annual #3, it was caused by an accidental explosion. I suspect that the CCA wanted the love triangle edited out of the story.
The Death of Ma and Pa Kent. When the story was originally published in Superman #161, the Kents were an elderly couple. But in Superboy #145, the Kents were transformed into thirty-somethings by a bottle of youth elixir. Thus, when the story explaining their deaths was reprinted in Superboy #165, their bodies and faces were redrawn to make them look younger.
Anybody aware of other changes? I seem to recall that the cover of JLA #2 was redrawn for the Archive Edition.