If I'm in chapter 13 but convert to chapter 7, can I get a refund of what I've paid to the trustee?

Only the amount of money that has not already been distributed to creditors.

The U.S. Supreme Court held in Harris v. Viegelahn (May 18, 2015) that chapter 13 debtors who convert their case to chapter 7 in good faith, are entitled to the return of the undistributed money held by the chapter 13 trustee.