So many things have changed since then. The big bad Ansem turned out to be the apprentice of the real Ansem, and also turned out to be an old man who invaded Terra's body and oh, he had a keyblade. Kingdom Hearts turned out to be an artificial fake Kingdom Hearts instead of the heart of all worlds and people. The world borders turned out not to be sealed in the end. Sora apparently was a human-shaped heart after becoming a Heartless, like Riku did after the fight with Riku-Ansem, all the way up until the beginning of KH2. Riku apparently learned of other worlds from Terra and not Kairi. The Heartless (specifically the symbol heartless) were originally created when a person lost their heart instead of the current explanation of heartless being created from a corrupted and disembodied heart. Originally Keyblade wielders did not just obtain Keyblades willy-nilly. Sora and Riku had to fight over a single Keyblade despite both being strong enough to wield one and both being threatened by the darkness (some of the presumed requirements for a keyblade appearing on its own). Originally Keyblades appeared to be sentient beings that chose their wielders in order to defend Kingdom Hearts and the world order, and a Keyblade could renounce its wielder if it believes the wielder is no longer working for its own interests.
Overall, characters' identities, backstories, and motivations changed either slightly or drastically while a large amount of the original lore was contradicted and revised by later games. The plot has also been revised, so some events in KH1 didn't exactly happen as the players were led to believe, or turned out to be meaningless in the end.