On the one the one hand, the Flash has pretty much the same schtick he's always had:
he's fast.On the other hand...
Let's start with one basic fact:
the modern Flash is not the same person as the Golden Age Flash. That, in my book, automatically disqualifies a character from being "least changed", as long as people like Bruce Wayne, Clark Kent, and Diana from Paradise Island are still around. The modern Flash is a different person that the Golden Age one. Other than becoming a villain, I'd be hard pressed to think of a bigger change then, well,
WHO YOU ARE.
Some will pooh-pooh my position as needlessly conservative. The character is "the hero who's the Fastest Person Alive", they'll say, and whether it's Pietro Maximoff or Johnny Chambers doesn't make a big difference. I understand their position; it's rooted in a view of the heroes as instantiations of mythostructural archetypes. But my reply remains "Piffel!"
Their position is based on the idea that it's the hero identity, not the private one, that matters. That the costume is what matters, not the person in it. Call me old school if you like. But I do not, and have not wavered one iota in my position that Bruce Wayne, Clark Kent, Hal Jordan, et al.-- those are their real identities. Those are actual people (um, I mean, "actual people in the comic books", of course!) who choose to put on costumes to fight crime and protect others.
If I hear one more person regurgitate the post-modern pap that "Bruce Wayne died when his parents were killed" or "Kal-El wishes to blend in with earthlings out of loneliness", I'm going to slap him. Real people have parents, develop crushes, go to school, pay bills, have sex; just like Bruce Wayne, Clark Kent, et al. We don't identify with Batman and Superman; we identify with "normal" people (!) like Bruce Wayne and Clark Kent and then thrill when they don their costumes to become Batman and Superman. Golden Age writers understood that; most modern writers don't.
Batman and Superman didn't decided to become Bruce Wayne and Clark Kent; it's the other way around. They had a choice of what to do with their powers and circumstances, and they made it. Don't deny them that humanity, that nobility. Without it, they aren't heroes, no matter what they can do, just emotionally crippled nutcases. Do that to your Marvel heroes, if you want, but leave your filthy deconstructionist paws off the DC heroes. Don't make Baby Adam West cry.
So...
The Golden Age Flash was Jay Garrick, a happy-go-lucky chemist and college grad, who acquired his powers as a result of an accident when he was performing an experiment.

The Modern Flash is Wally West, the nephew by marriage of Jay's unrelated successor. Wally isn't a college grad. Last I heard, he was a mechanic, although for most of his career (including right now), his money just seems to come from nowhere, or maybe the Speed Force. Like it forms into his costume, maybe the Speed Force magically fills up Wally's bank account, as if it were
Zephraim Cochran's Companion. Wally, at least as he's been written outside the JLU, is anything but "happy-go-lucky".
The Golden Age Flash wore a jogging outfit and a shiny Mercury hat; he did everything with a smile. The Modern Flash wears skin-tight Speed Force (tm) brand spandex that covers his head and his hands, and a cowl with wings over the ears; he does nothing with a smile.
The Golden Age Flash, still pretty much unchanged and in great shape, continues to fight crime as a member of the Justice Society, having new adventures, still smiling and serving as an example to the younger generation of heroes. In fact, he served as a mentor to Kid Flash, when the Modern Age Flash, rather than mentor the kid, shipped him off to be raised by an elderly dynastic relative. I mean,
the second time the Modern Age Flash, rather than mentor the kid, shipped him off to be raised by an elderly dynastic relative.

The Modern Flash, who's famous for stories that get nowhere fast or that run in circles, was so tapped out as a character that they killed him off and brought him back, and almost nobody noticed. Now, desperate for something to do with the guy, they've made him the head of a thinly disguised
Incredibles knock-off, so that if we're not interested in him, maybe we'll at least be interested in his kids.
And,
like most parents, he doesn't smile a lot.The Golden Age Flash's powers came from science. Yes, really really
bad, absurdly inaccurate science. But science nonetheless.
The Modern Flash's powers come from a New Age mystical source, called ...
- the Green?
- The Lady of the Lake?
- The Morphogenic Field?
- The Red?
- The Ether?
- The Hoary Hosts of Hoggoth?
- The Clear?
Gosh, why can't I remember it, it's something really clever like, oh, yeah:
"the Speed Force".
Call me old-fashioned, but I'll take Bad Science over Bad Mysticism any day.
In short,
the only thing about the Flash that's remained the same since the Golden Age is the name of the hero and the fact that he runs fast. I guess for some people, that's enough; I'm just not one of them.
How much do YOU think the Flash has changed?