From ACTION COMICS #267 "The Three Super-Heroes!":
Honestly, I can see the bottom blurb in the yellow box has been rewritten (and the pages recoloured!), but I can't actually read the earlier page as-posted here.
I found it a bit hard to read too. Here is the original text and the changed version:
Original Blurb:
... and what is their purpose in helping her hide the secret of her real identity? The answer is even more fantastic than she could possibly guess, and leads her 1,000 years into the future, to a stunning surprise finish in the amaze-a-second adventure of ... The Three Super-Heroes!
Reprint Blurb:
... and what is their purpose in helping her hide the secret of her real identity? Right! They're Cosmic Boy, Saturn Girl and Lightning Lad, of the Legion of Super-Heroes! And this is the fantastic tale of their first trip from the 30th-Century to see the Mighty Maid of Steel! The reason is told in this account of ... The Three Super-Heroes!
Original panels:
Supergirl "You're the Cosmic Boy ... Saturn Girl ... and Lightning Lad he knew!"
Lightning Lad "Not the ones he knew, although we have the same names ..."
Saturn Girl "We are the children of the three young super-heroes who befriended Superboy! We are carrying on the Legion's traditions ..."
Reprint panels:
Supergirl "You're the Cosmic Boy ... Saturn Girl ... and Lightning Lad he knew!"
Lightning Lad "You're hitting on all cylinders, Supergirl!"
Saturn Girl "We've admired Superboy ... and you! That's why we made this trip!"
As you can see the original was working from the idea that since Supergirl was a generation later than Superboy, so the Legionnaires from the future had to be as well. The next time Supergirl meets them this is just dropped and this reprint takes it out all together. Hence Korbal's title "The First Legion Retcon..."