Can a believer lose a crown that is already laid to his account?

In your book In Light of Eternity, you say we can lose our crowns and quote I Corinthians 9:27: "But I keep under my body and bring it into subjection: lest by any means, when I have preached to others I myself should be a castaway." I have understood the word "castaway" translated as rejected from the competition. I can see this as not receiving a crown that would have been available just as not receiving a reward that would have been available, but losing a crown and having it taken away from you, a crown that is already ...