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.
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.