When I think of how the Silver Age changed comics, I usually think of Julius Schwartz, then Marvel, putting their distinctive stamps on the superhero genre. Rereading the Silver Age month by month, however, it’s obvious that even old, established characters were changing, particularly around 1958-9, which is when many of the Showcase TPB collections start (otherwise I’d be out of luck reading this stuff).
Wonder Woman #98, for instance, has Ross Andru and Mike Esposito taking over the art from WW’s co-creator H.G. Peter (they’d continue as the art team for a decade). Writer Robert Kanigher immediately retcons Wonder Woman’s origin in “The Million Dollar Penny,” arguably the first Earth-One Wonder Woman story. It’s contemporary, rather than set in WW II, and Diana doesn’t meet Steve until after she’s already become Wonder Woman and started for Man’s World. In William Marston’s original story, Hippolyta didn’t want her daughter to leave; in the retcon, the queen’s worry is that she’ll be biased in her daughter’s favor and choose her over a worthier Amazon. The solution is to have all the Amazons wear Diana masks (Kanigher loved having Wonder Woman fight doubles of herself) so Hippolyta can’t tell who’s who.
The next issue gave us a different origin for Wonder Woman’s Diana Prince identity. In #105, we get a different origin for the Amazons (retreating to Paradise Island after their men are all killed in wars) and meet Diana as Wonder Girl (the story that introduced the term “secret origin“).
None of this improves on Marston (except, perhaps, dropping the idea Diana’s leaving the island for love) but it did serve to put Kanigher’s stamp on her for the rest of the Silver Age. And it would have a ripple effect — the introduction of Diana’s younger self led to Wonder Girl joining the original Teen Titans and to the later Wonder Family feature in Wonder Woman.
Aquaman’s new origin was a lot more impressive and a bigger retcon (though possibly not the first Earth-One Aquaman story). The Golden Age Aquaman was a scientist’s son endowed with the ability to live underwater. Adventure #260 retconned that into the story we all know, that Arthur Curry is half-human, half-Atlantean. 
This Robert Bernstein/Ramona Fradon story is light years better than the original. While it didn’t immediately improve Aquaman’s forgettable Adventure Comics backup series, it made possible the introduction of Aqualad, Atlantis, Aquaman’s role as king — pretty much everything that makes Aquaman more memorable than, say, the Fin or Neptune Perkins. While I’m not a believer in definitive versions of characters, it’s hard to improve on this one (Peter David tried but unlike my colleague Greg Burgas here, David’s run didn’t do much for me).
Green Arrow also had a 1959 origin retcon, with art by Jack Kirby during his short run on the Emerald Archer. Oliver’s original origin involved being stranded with Roy Harper on a mesa in the Southwest, eventually using archery to fight off criminals robbing an archeological site. The retcon, as everyone reading this probably knows, has Oliver stranded on Starfish Island and teaching himself archery to catch food.
Like Aquaman’s, it’s a solid origin that’s never been replaced (even the CW’s Arrow went with it, though in considerably mutated form) though it’s certainly been tinkered with. Mike Grell, for example, showed Ollie studying archery before his sea voyage, which is more plausible than his miraculously becoming an ace archer after starting from scratch. The improved origin did not, however, improve GA’s C-list Silver Age adventures at all. That took growing a beard and turning radical, which was years in the future.
And then there’s Superman, who didn’t get an origin reboot but did get a lot of new ideas around this time. In 1958, Otto Binder introduced the Arctic Fortress of 
Solitude in the story that kicks off the Showcase four-volume set (and also has Batman playing a joke on Superman. I love that), just a couple of months after he’d introduced some one-shot characters called the Legion of Superheroes. Right after the Fortress we got Brainiac and a bottled “Krypton city,” the Phantom Zone, Bizarro (Stan Lee can puff himself up about how the Hulk was a modern day Frankenstein’s monster, but in the first Bizarro story in Superboy #68, Binder nails it) and of course Supergirl. That’s an impressive spurt of creativity.
I’d like to draw some conclusion from this, but I don’t think I have one. Just that it’s cool to see comics history developing before my eyes.
#SFWApro. Images top-bottom by Ross Andru, Ramona Fredon, Jack Kirby, Curt Swan.
“has Oliver stranded on Starfish Island and teaching himself archery to catch food.”
Right now I’ve got pictures in my head of him shooting arrows at ordinary starfish. 🙂
Undoubtedly excellent training for future encounters with Starro.
The JLA are facing off their first and worst enemy, and he just struts in and says, ‘hey guys, I got this.’
A special five-warhead arrow for pinning down all five legs at once!
Ollie actually did face Starro in JLA 65 (one of several villains manifested by TO Morrow). It didn’t go well.
When I read these comics, I sometimes think about how it would be cool to be living in an era where comics were a much larger business and there was a lot more variety in the genres being published. But then I remember there would be no internet, only 3 channels, and all the ways culture has changed since then.
I actually think we’ve come around to the point of much greater variety again, especially if we factor in reprints and digital comics.
While I do enjoy the huge range of TV viewing options, there are moments I do grow nostalgic from when I had three channels, PBS and didn’t have to spend so much time thinking about my selection (the paradox of choice, as they say).