Are the cornerstones the same color as the main blocks -- do they add to the design? If they do, and it would look entirely different, then I would take it apart and add them. I would not, however, add them OVER the material that is already there. It would be very noticeable and IMHO ruin the effect you already have. You don't quilt like a beginner, really. It's going to be beautiful either way you do it.