What changed in Dragon Ball Z Kai?
Dragon Ball Kai, released abroad as Dragon Ball Z Kai, premiered on April 5, 2009 for the twentieth anniversary of Z. Kai means revised, and the revision was editorial: Toei took the original Z footage, remastered it in high definition, redrew damaged frames, commissioned new openings and endings, had the surviving cast re-record the dialogue for performances closer to the manga, and cut most anime original material so the story would move at the manga's pace. The result tells chapters 195 to 519 in 159 episodes in the Japanese broadcast, against Z's 291. The Garlic Jr. arc, the fake Namek, and the Other World Tournament are simply gone.
The first run also ended early in a way that confuses every episode guide. Kai was scheduled for 98 episodes through the Cell arc, but the finale, episode 98, did not air in the original run because of news coverage of the 2011 Tohoku earthquake and tsunami; it reached viewers as a home video release. The Buu arc arrived years later as a separate production, from April 6, 2014 to June 28, 2015, and it exists in two lengths: produced at 69 episodes for international markets, where it is titled The Final Chapters, and edited down to 61 for the Japanese broadcast, which is why the series totals 159 episodes in Japan and 167 internationally. The tighter Japanese cut exists because the time slot was promised to a new series: Dragon Ball Super, which premiered a week after Kai ended.
The strangest change was the music. Kai launched with a new score by Kenji Yamamoto, the composer of the Budokai games. On March 9, 2011, Toei announced that Yamamoto's music infringed the rights of unspecified third parties, removed him, and replaced his score, for the last episodes and for reruns and later releases, with Shunsuke Kikuchi's original Z music, music written for different cuts of different episodes and laid in as an emergency measure. The Final Chapters moved on to a third composer, Norihito Sumitomo. So Kai, the edition built to be definitive, carries three scores across its life, and its earliest home video pressings preserve a soundtrack Toei itself withdrew.