I was thinking first of Pendragon, where 4e and 5e are based in the same setting, have many of the same rules, but are actually radically different games. One is a fantasy adventure game where you can be a knight, a rogue, a wizard, whatever, set in fantasy-britain; the other is a game where you play Knights, PERIOD, and live through the Arthurian Saga.
But of course there are many games where the "essence" of the game is changed, beyond any question of the rules, from one edition to the other.
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