I know if they remove the Supreme Kais, he will become the original Buu again. But, since the Good Buu and the Z Fighters were already removed from Super Buu's body, turning him into Kid Buu, then the Grand Supreme Kai and the Supreme Kais are definitely still inside Good Buu's body! But wait a second, if they remove the Supreme Kais from Good Buu, doesn't it mean that he will become Kid Buu again? Or he will split into the Supreme Kais and not having back the "Buu" body? Or, the Supreme Kais were just simply not exist inside Good Buu?
Remember that Fat Buu was removed from the Fusion/Super Buu. Both of the original Kais had long since been fully absorbed/consumed, and Fat Buu was the only thing left. Once Goku removed Fat Buu (also Supreme and Ultimate Kais), then Kid Buu (original) is what we are left with. If they had still been inside of Super Buu, then Goku/Vegetta would have seen them near Fat Buu.