Who has changed the least?
Only a select group of Golden Age heroes (Superman, Batman, Wonder Woman, Green Lantern, Flash, Aquaman, Green Arrow, Black Canary) are still around. And I don't mean "retrieved" characters like Alan Scott, Jay Garrick, or any of the JSA-style inheritors. They are a different animal entirely from the Underoos Icon Gang.
So let's pull our Underoos out of the drawer and examine them over the next few days ... just to see how they're holding up.
Well, he looks the same except for some minor vagaries of costuming. But he's changed alot.
He's added a host of powers (many of which, such as superventroliquism, were mercifully shed along the way), and the powers that have been consistent in kind have increased dramatically in degree.
His personality, on the hand, has changed in kind. The Golden Age Superman was almost anti-authoritarian. As a reaction to the powerless people felt during the Depression, the GA Superman broke into people's houses (including the Governor), beat the snot out of normal humans, hung them from flagpoles, grabbed unsympathetic employers and forced them to experience the deplorable conditions of their workers... . Superman (no matter what Frank Miller might think of him), has never been about supporting authority, but about having the power to circumvent it.
Yet, nowadays, we not only respect and envy that power, we fear its misuse. So Superman, most powerful of heroes, has had to evolve into the very image of He Who Would Not Abuse His Power, and come to be an authority figure himself. John Byrne (no matter what you might think of him) understood this tension and very early on he cleverly had Mayor Berkowitz use Superman to circumvent regular, slow justice and arrest Lex Luthor on the spot ... after saying he'd "deputized" him, of course.
His larger "secret identity" schtick is still very much intact. Clark Kent is still mild-mannered (I mean, as much as an investigative report can be) and his difference in personality from smiling, confident Superman is still his best disguise. But the two-and-half-sided triangle between Clark, Lois, and Superman is no more.
That's a big change. Superman used to symbolize men's need to be loved and admired not only for their aggressive confrontational selves, but for their quiet nurturing selves as well. And, I might add, some woman's refusal to do so, particularly strong capable ones like Lois. Despite professing to want "sensitive" men, women usually really go for a confident macho guy. How else would you explain that so many comic-reading women who would call themselves feminists are googly-eyed over Hal Jordan, of all people? I don't know a single comic-reading gay guy who goes googly-eyed over Hal Jordan; only women. Comic-reading gay guys go googly-eyed over guys who are confident yet secure enough in their masculinity to wear yellow balloon pants and red neckerchiefs. And breakdance.
Anyway, while that idea of being loved that way used to be presented through Superman as nearly impossible, his marriage to Lois (in which she loves both the Clark Kent Him and the Superman Him) is now shown as an ideal. In all fairness, then that's not a change: that kind of love was presented as ideal and the only kind worth having in the Golden Age, too. That's why Superman never accepted Lois's advances; if she was too good for Clark Kent, then she wasn't good enough for Superman. It's not Superman who changed; it was Lois.
It's a pity they never really showed us that change; they were kind of suddenly, you know, married.
In any case, how much has Superman changed in your eyes?