This paper gives a unified account as well as a prediction of the ongoing global paradigm optimization in Japanese voice, in light of Probabilistic OT (Boersma and Hayes 2001, among others). In the grammatical category voice in Japanese, a global language change that reorganizes the conjugation paradigm is currently underway. The process includes three variable forms: sa-Insertion, ra-Deletion, and re-Insertion. First, I characterized the chronological change of the distribution of each phenomenon employing a large-scale corpus. Second, I formalized the changes in standard OT terms; in turn, I modeled and predicted the change within the framework of Stochastic OT. The results show that the gradient approximation of the ranking values of each constraint caused the emergence and the gradual increase of innovative forms, and that the overall change in voice can be explained by the dynamic interactions of a small set of constraints. Furthermore, evolOT (Jäger 2002) predicted the changes of variable forms in an S-curve manner that is consistent with the observed data.