Kaneto Shiozawa, the original voice actor for Zato-1, died from injuries sustained in a tragic accident. Takehito Koyasu would succeed him for that role.
Iemasa Kayumi, Slayer's voice actor, died on September 30, 2014. He has thankfully finished the recording for Xrd, but future appearances of Slayer would require a new actor, or worse, he could get killed off like Zato.
The Character Died with Him: Kaneto Shiozawa, the original voice actor for Zato-1, died from injuries sustained in a tragic accident. After that, Zato "died" and his body was taken over by Eddie and he has been voiced by Takehito Koyasu... until Xrd, when he suddenly went back to being Zato-1, and still with Koyasu as the voice actor.
So far, Baiken has three. The original one, used exclusively in The Missing Link was Satomi Koorogi (the voice of May), who previously was the voice of Sakura (but not Mayo Suzukaze, who voiced the actual Kenshin Himura, off whom Baiken was based). The second one is Sudo Miho, whose deep voice acting was used in a good portion of Guilty Gear games, at least up until Accent Core, which had Chizu Yonemoto instead.
Potemkin originally was Hideyuki Anbe. From Guilty Gear X and onwards, he is voiced by Takashi Kondo.
Chipp Zanuff has three. The original one, used exclusively in The Missing Link was Takuya Morito. The second one is Takeshi Miura, who was from X all the way to Accent Core. In Xrd, he is now voiced by Yoshihisa Kawahara.
Kliff Undersn was originally voiced by Hatsuaki Takami. From X onwards he is voiced by Shigeru Sakano.
Justice has three. The original one, used exclusively in The Missing Link was Takuya Morito. In XX she is voiced by Yumiko Ogawa. The third, Wakana Sakuraba, only voices her in the Accent Core Plus story mode.
Bridget was originally voiced by Hiroko Konishi. After she abandoned voice acting, Yukiko Kato too over the role in Accent Core.
Robo-Ky originally had Kantaro as his Battle Voice and Takeshi Kusao as his Story Mode voice. In Accent Core, Takumi Inoue voices him.
Testament originally was voiced by Takami Akkun. His voice actor from X onward is Akira Kobayashi, who uses aliases every single game. So far there's been Konishi Kobayashi and Kobayashi Katsuragi.
The most notable one is Sol, who for the longest time was voiced by series creator Daisuke Ishiwatari before Joji Nakata stepped into the role for Overture and Accent Core.
And now the English dub of Xrd has followed suit. Ky went from Liam O Brien to Sam Riegel, and Sin went from Yuri Lowenthal to Lucien Dodge. Funnily enough, Liam and Yuri returned but only to voice different characters entirely: Axl and Bedman respectively.