Batman replaced by Dick Grayson... Green Lantern and the Flash replaced, too. Did you catch all of that news? It happened in 1951. In Batman #66, Dick Grayson dreamt of a future in which Bruce Wayne stepped aside so that Dick could become the new Batman. A few months earlier, the original Flash and Green Lantern made their last appearances of the decade.
When I started reading DC comics in the early Seventies, I saw three parallel timelines unfold at once. The new stories told the first-run adventures of the JLA, written in the Seventies and printed in the Seventies for readers in the Seventies. By chance, the first JLA comic I bought was the same one that introduced Libra, who reappeared in Final Crisis.
But at about the same time, DC was reprinting Golden Age stories. The very oldest comic that I kept ahold of was published in 1972, but showing the then-thirty-year-old origins of Wonder Woman and Wildcat. Meanwhile, many JLA issues had older features in the back, either from the original JSA era or JLA stories from the prior decade. And moreover, the JSA appeared in crossovers with the JLA. The distinct art styles immediately told me which era a given story matched, and I preferred the clean lines and sleek art of the newer ones. I also found the older characters harder to get to know because the coverage skipped around, and moreover had less of a tendency to discuss the heroes' identities. I felt like I knew Elongated Man after reading probably no more than fifty panels with him speaking, whereas Hourman seemed like an outsider who showed up and fought without my understanding him. In truth, there probably wasn't that much I knew about the Elongated Man, either, but at least I knew that he was named Ralph Dibny.
Superman, Batman, Wonder Woman, Green Arrow, and Aquaman outlived the end of the Golden Age. If a few more of the JSAers had, as well, there probably never would have been the idea of such distinct eras. But as it was, only three heroes nailed down enduring solo titles while the rest faded away. And when editor Julius Schwartz decided to fire up the superhero machine again, he decided to reinvent some of the JSAer with new names, faces, and outfits. Different origins, too, with Green Lantern, Hawkman, and the Atom being reborn as science fiction heroes, which the Golden Age versions of them were not.
I always thought of the Golden and Silver Ages as having a blank decade between them (the time between the last original-run JSA story and the first JLA story), but the truth is that the transition was more gradual. Superman and Batman "met" in Superman #76 only one year after the last JSA adventure, effectively rebooting their personal continuity (forgetting their past mutual membership in the Justice Society and starting a continuous friendship that was not reinvented again until 1986). Just one year after that, a 1953 story titled "The Manhunter From Mars" appeared in Batman #78, reading like a rough draft of J'onn J'onzz, the eventual charter member of the JLA.
But if you want to pinpoint the first "rebirth" in DC superhero comics, it was the Flash, who was reinvented in many ways (although the lab-accident origin story remained, with some tinkering). Unlike everything else the Flash did, this happened slowly -- Barry Allen did not inherit the Flash title until 1959, almost four years after his first of four appearances in Showcase. But this character was infectious like none had been since Superman's own debut in Action #1. The success of the new Flash led directly to the new Green Lantern later that year. Barry Allen basically made Hal Jordan.
Fifty years later, Hal returned the favor. Thanks to a fabulously successful reinvention of Green Lantern in the hands of Geoff Johns, the resurrection of Barry Allen looked like an obvious decision, business-wise as well as aesthetically. If Hal hadn't soared to the top of the sales charts (outselling, for a time, any other solo character from DC or Marvel), Barry Allen might still be a trail of ashes in the Anti-Monitor's headquarters.
Geoff Johns obviously sees a lot of past in the future. After a strong run on JSA, he's penned both Flash Rebirth #1 and Blackest Night #0, which look more than anything like two episodes in one story in which our heroes beat death. Not just by escaping a death trap, but by escaping death itself. The JLA took four of the Golden Age's surviving heroes and paired them with three Silver Agers. We now know that those three Silver Agers will all come together again in Green Lantern #44. The reunion doesn't look like it will start off very friendly, but with these heroes, there's always time for a happy ending.