The Other Darrin: Rebecca Forstadt and Wendee Lee took over the roles of Mihoshi and Kiyone, respectively, in the English dub from Tenchi in Tokyo onwards; Mona Marshall replaced Petrea Burchard in the role of Ryoko from Tenchi GXP onwards.
While Wendee Lee took over the role of Kiyone, the original actress - Sherry Lynn - was still voicing Sasami. This was because doing Kiyone's deeper voice was hurting Lynn's throat; her natural voice is actually very close to Sasami's.
Every major character in the series has been his with this at least once. This is because GXP was not dubbed by the series original dubbers, but Funimation, who used an all new cast. Then, for Sasami: Magical Girls Club, the few characters carried over from other works got newer voices again.
Washu's played by lesser known voice actress Kate T. Vogt for the vast majority of the series. This ends with GXP, where she is voiced by relative unknown Jessica Gee, who, well, let's just say fans were not impressed. Luci Christian is Washu's current voice come Sasami: Magical Girls Club.
Tsunami was not played by Sherry Lynn in Pretty Sammy, but Lara Cody, despite the fact that Sherry Lynn was still Sasami.
Tsunami and Sasami in both the Japanese and English versions. Also a bit of a meta-example, not only for fitting the trope itself but for fitting its title literally thanks to Sasami and Tsunami's fusion link.
What Could Have Been: Washu was originally planned to be a villain in the OVA and Kagato was just an illusion created by her, but then the show's creators found she was too fun a character to kill off and kept her around.