The Story: "The Day Superman Became The Flash!" by Edmond Hamilton and Al Plastino.
Superman gets an urgent message...from the Justice League of America!
On the planet Xann, for instance, Kal-El would be miniature compared to that planet's giant inhabitants. Turns out that young Kal, still with Superman-like powers, would again become a costumed superhero, similar to The Atom! Jor-El tries again, seeing what would happen if Kal was sent to the all-water planet known as Valair:
Roll Call: Superman, Batman, Aquaman, Flash, Green Arrow, Atom
Notable Moments: What a bizarre story! It has become known over time that during the early 1960s, DC was having a lot of trouble coming up with stories for its big three characters, and this story certainly reads like someone was desperate for inspiration. "Hey, wouldn't it be fun to see Superman act like the other Justice Leaguers?" seems like a fine enough proposition, but the whole "Jor-El peering into the future" conceit is so ridiculous and contrived that (IMO) it clocks an 11 at the Goofy-Meter.
And while I'm not complaining of course, it feels strange to A)have Aquaman as the spokesman for the League (since when?) and B)have the other JLAers never utter a single word in the story! They show up and then just stand there, as Aquaman drones on and Superman gets all misty-eyed, as he always did when talking about Jor-El and Lara.
This story marks the JLA's fourth-ever appearance outside their own title; other than helping out Adam Strange in Mystery in Space #75, the team was generally written into members' solo books as a colorful mob who basically just sat around, commenting on the action. At least here some of them got a nice trip to a tropical island!
One final thing: I wonder why the cover and title of this story focuses on Superman becoming The Flash only? I'd say "The Day Superman Became The Justice League" has a little more snap to it.