This isn't the first time one of Yao's characters in One Piece gets a replacement as Wataru Takagi replaced Yao as the voice of Jango starting in episode 1,121 due to Yao's poor health. Aside from Franky and Jango, Yao also voiced Bon Clay.
In his official Twitter account, Yao stated, "A sad farewell has come, a separation with a person I have spent nearly a quarter of a century with." He also added that the gap is now open between his "ideal" Franky and himself, and he decided to "depart from the Thousand Sunny for now." He also expressed sadness and frustration, but added that a "su~per~ replacement" is being prepared.
[Source: Anime News Network]