What is the reason for the change in Nami's Seiyuu for a short period of time (Japanese Dub)?
According to Nami's Wikipedia's page:


  In the Japanese version of the entire One Piece anime series, and later spin-offs, Nami has been voiced by Akemi Okamura. In 2001, Okamura temporarily left the series due to her pregnancy; Wakana Yamazaki acted as a substitute for episodes 70-78.