I mean like Daveym shows in his post above, there are different interpretations of that. But whether Post Crisis Superman was E1 Superman changed, or a different character, is really splitting hairs.

The fact is that most of Earth 1 Superman's history was ditched so he's a different character in execution either way. We see all of his villains like Luthor, Brainiac, Metallo, Bizarro, all get reset. So even when you can say his history with the Justice League is still intact from Earth 1, it's Earth-1 history minus any episodes that featured any of those Superman characters. Which is why we got JLA Year One to show what the League's history was like without Superman, Batman, and Wonder Woman. Similar to Earth-1 but different.

Also, Post-Crisis Superman came from a very different Krypton than Earth-1's. There was no Supergirl, Krypto, Zod, Phantom Zone, Kandor, Argo City, at least not for many years and most of the iterations that came later were either imports from another universe or if they appeared in the main DCU, it was after more reboots.