User blog comment:B214/Question & Answers 5/@comment-207.96.205.2-20160517060930/@comment-1359322-20160518012936

Devils like Grayfia, Ravel and Kuisha has received the Evil Pieces before, their soul never got changed. So no the Evil Pieces doesn't change the soul into a Devil. Yes the soul is connected to the Evil Pieces but that's all, the original soul is kept as it is.

In fact if the Evil Pieces changes the soul into a Devil, then the Regulus Nemea won't be a Sacred Gear or Longinus anymore.